I’ll take another stab at it!
Yes, if your FCP is hooked up to an IO and he’s using 8 bit, he’s probably using the Aja “8-bit Uncompressed codec”. But even if you don’t render it to that codec, FCP should be able to no problem.
Does the file you render look good on your PC? If not, it’s probably a setting somewhere in AE (probably the render setting or ouptut module). If it looks fine to you, there must be a problem in going from your PC to Mac.
If it is a problem with AE, try using a different preset (like PSD sequence like someone said) and see if that works. You will have to bring that image sequence back into AE and render out as a Quicktime though to work in FCP. If that works though, you’ll be able to trace the problem back and fix it.
If none of this works, write back….I’ve got lots of render time to kill today!