good question. i don't know how many sd quattro users there are on rff.
i've been seriously considering the sd quattro, and was thinking of getting the bundle with the 30mm and eventually getting some other primes. but you bring up a good point about the OS - it can help offset the poor high iso, with static scenes at least. i hadn't thought about it that way. on the other hand, dreams of using foveon in low light may be a path to disappointment.
although with the merrills, high iso b/w isn't bad.
i'd also be curious to hear from someone using the 17-50 with the sd.
ETA: i found this thread with sd quattro/17-50, the user seems happy with it.