1% is good enough. I'm doing RMS current, and I'm sampling 4 channels on a 740uSec interrupt. I have to be able to do all A/D + squaring + avg + sqr in
that interval. I'm already "interleaving" - doing the math on the previous sample while the A/D is converting, but I can't quite get everything done in the allocated period - even while running 40MHz. I don't have to gain too much to make it work.

I would definitely be interested in your code.