x264-> is there a optimized Athlon64 x2 (VFW-)Build?

as subject says, I have an Athlon 64 x2 dual core cpu. Is there a special compiled version for those cpu? And does the vfw-Version support multithreading? Btw, the cli-version only get used up to 80% of cpu when using --threads 2 option?

Possible reasons: - Fastcall calling convention. Parameters passed via registers instead of in memory via the stack. - More general purpose registers, so again less memory usage.

a little searching wouldn't hurt, now would it?