I've used 6 over 250 (roughly 520nW/m) as my standard alignment on 456 since about 1978, often at 15ips.
That's what all of Relish is, on the multi-track, for example.
On 499 or higher output tape, I go another 3 dB, but in truth I like the sound of 456 better.
Totally true about A800's vs the 827 and 820.
I'm not a huge 16 track fan, the improvement in sound is real, but subtle enough to me to not make up for the lack of tracks... and if you end up tranferring to digital anyway, you;re going to LSOE more than the 16 track gained.
Also, I'd be happy to blind A-B an MCI/Sony 16 against an A800 24... I'd wager the Studer would win in a walk.