On my rig, bouncing UAD stuff at the highest buffer on the Fireface (2048) gets them to render at a rate close enough to the native stuff that they're no longer necessarily the reason for an export slowdown logjam. Faster than the RX DeCrackler, for example (which I'm typically using to pre-treat tracks before editing, so that takes them out of the export queue. Much faster than at 1024. I generally edit at 512, as that's how they say the UAD Satellite works best, so a lot of switching goes on.
I guess I should add, based on the OT, that exporting at these rates, I'm experiencing no noticeable difference between N4, N5 and N6.
*Edit* Geez... I just did a search looking for the original "change your buffer" thread, and found the other where we already talked about all of this stuff. Enough from me on this...
N6.0x 32bit, C4, Halion, UAD-2s, Fireface 800, MC Control, Mac(s) 10.8.x, 16G