Hello, Ted--
> How close can you fake it with a pair of supercardioids?
This question opens up a set of several large issues all at once. Coincident supercardioids let you capture a wider angle of incoming sound than coincident figure-8s can; also, with condenser microphones at least, you will generally get fuller low-frequency response from supercardioids than from comparable figure-8s. Depending on the type of music you're recording and the type of hall you're in, those considerations can be crucial. The "down side" is that with supercardioids the stereo image in playback will emphasize the center artificially to some extent, though not as badly as coincident cardioids will.
More important to my ears is that, while supercardioids pick up less reverberant sound than figure-8s at any given miking distance, what they pick up contains a different, and to my ears less favorable, mix of reflected sound than the mix which figure-8s tend to pick up.
Let me try to explain that a little. Both types of microphone are technically bidirectional--they each have a front lobe and a rear lobe, with the rear lobe being in opposite signal polarity to the front. The difference between the two patterns, of course, is that in a figure-8 microphone the two lobes are equal while in a supercardioid the front lobe is emphasized--it is considerably more sensitive, and also broader, than the back lobe. That extra broadness of the front lobe is what allows a pair of supercardioids to capture a wider array of performers on a stage than a Blumlein pair can capture, but there is both a direct and an indirect price to be paid for that ability.
Keep in mind that reverberant energy coming from behind the microphones is very likely to have travelled farther, to be more diffuse in character (with fewer identifiable, discrete reflections) and thus less likely to distract from the main image, and to have undergone a greater degree of high-frequency absorption both from multiple bounces off of hall surfaces and simply from the greater distance that the sound has travelled through the air before reaching the microphones. Consider this: You can take two microphones with any different directional patterns (say figure-8 and supercardioid), and place each one at a point in the hall where the ratio of direct sound to reverberant sound is exactly alike between the two of them (e.g. a figure-8 would need to be closer in than a supercardioid). But if you do that, even though the amount of direct sound and the amount of reverberant sound will be the same in the two microphones, the one which is farther away will be picking up more of its quota of reverberant energy from the front and sides of the stage rather than from the back of the hall. Those are, on average, shorter path lengths for the sound waves. Thus it will be picking up more early reflections and a brighter and/or harsher selection (if you will) from the total body of reverberant sound energy that is available in the hall. Generally, that's not preferable.
Here's the thing: I've recorded with supercardioids (or something between super- and hypercardioid, which is what most such microphones really are) more than any other single pattern, so obviously I think they're one of the most useful types of microphone. But that has been largely from necessity--because of the low-frequency issues, the geometric limit of a 90-degree pickup angle from a Blumlein pair, and for another big reason which is not about microphones but about performing spaces. Let me just say that I am quite unhappy, one way or another, with the sound of most halls that I currently hear music performed in. I do not think that those of us in the U.S.A. live in an age of really good architectural acoustics for music; we live in an age of "multi-purpose" spaces that aren't really good for any particular purpose. I've lived in New York City for the past 23 years and the pleasant, naturally well-balanced spaces for performing music here are few, far between and expensive to rent.
Poor hall acoustics is the big elephant in the living room that no one talks about and that everyone is so used to that we just walk around it and work around it as if it were nothing worth mentioning. But a Blumlein arrangement is hopelessly dependent on favorable acoustics in the way that a sailboat is dependent on the wind blowing in the right direction for your journey--and acoustics that allow you to use Blumlein stereo to best advantage are unfortunately rare.
Finally, there's one more big factor to consider that isn't strictly about microphones or patterns, but about overall geometry, balance and clarity. As I've said, a Blumlein stereo array is limited to a 90-degree front pickup angle, and when you need to record something that is wider (from the microphones' point of view), you need to choose another pattern and/or another technique. The question I'd raise is whether coincident stereo is really the best choice at all when a performing ensemble is so wide that you can't fit it into a 90 degree pickup angle and keep a good reverberation balance. My concern is not so much a matter of angles as it is of the range of distances between all the performers and the microphones.
Most "main microphone" setups (coincident, spaced or in-between) are placed closer to the front of the stage than most members of the audience are sitting, even in the good seats. This distorts the perspective of the recording somewhat, because relatively speaking, the closest performers are closer to the microphones than the farthest performers are by a far greater ratio than is true for the audience. This affects the musical balance both in terms of volume and clarity. The string section tends to overwhelm the pickup, with the front desks being perhaps 5 to 10 times closer to the microphones than the back desks of the same section of players. If there is a soloist in the front (e.g. in a piano concerto) the soloist may literally be 20 times closer to the microphones than the people at the end of the back row of the orchestra. Twice as close might make sense from a balance standpoint, but certainly not 20 times as close.
Say what you will about phasiness and vague imaging in spaced microphone arrangements, but at least they are not the worst case in this regard; coincident arrangements (of any kind) are the worst case. If the hall acoustics and your choice of recording method and microphone pattern(s) don't let you get some appreciable distance from the front ranks of the performers, you will get sound that is too loud and clear from them and too fuzzy and distant from the players in the back--who are (or should be) just as good musicians as the ones in the front.
I'm sorry that again this is a very long message, but I think that it's a mistake to treat microphone patterns as abstractions when in practice they bring all these other concerns into play.
--best regards