I've done quite a few recordings on 499 @15 with no SR. IME it really depends what kind of material you're talking about... for rock'n'roll and similar the noise is really not a problem and I never noticed much difference vs. 30. If you have very quiet pieces, it may be. I really love the way cymbals sound at 15, electric guitars... it does something to the high end that's extremely cool for certain material.
As far as running at +9... I've never found that Studers deal with that particularly well at 15... they seem best at +6, you might be able to get away with +7 or +8, however Ampexes seem to take +9 fantastically well at 15, and MCI's somewhere in the middle. My general experience with Studers is that they are magical when operated within reasonable specs but they aren't as great when you really push them, as opposed to some other decks. But, YMMV, and different machines even within the same model sound different, etc.
But as a very general statement... 499 @15, no SR, no problem.