I cloned Lukejr's latest with Zeus support and increased the max clock to 600 in the code.  Just from some testing I could do this morning, anything above that magic 381 gives VERY erratic results.  I raised it to 425 at one point and all I got was rejects.  Dropped it back to 400 and it returned to working like it did before 425.  I was getting a little higher error rate (8%), but between the error rate and rejects I wasn't gaining any more speed at 400 than at a lower speed like 381.  I dropped it back to 390 just now and will let it run for a couple hours to see where it's at.

I remember changing the max clock in Darkwinde's fork of bfgminer but that was before his re-base to Lukejr's 4.3.0.  Unfortunately I don't remember if it had the same behavior or not.

:EDIT - even 390 is showing 32% reject and 9.6% hw error....not looking very good for even 390.