I bought the new Pro version because X3 would not export a usable audio file. The Pro version also failed to export untill I increased buffers from 256 to 512. Now both versions are working.
Is 512 a reasonable requirement?
Now that both versions work are their any major advantages to use Pro vs X3c other than Breverb?

The current version of Pro is way more evolved and has benefited from numerous bug fixes.
 
Optimal latency depends on many factors including the plug-ins that are used, project complexity, computer CPU speed, virtual instruments, and more. I just set it as low as possible until things mess up, then I raise it a notch 

deanhoward
I bought the new Pro version because X3 would not export a usable audio file. The Pro version also failed to export untill I increased buffers from 256 to 512. Now both versions are working.
Is 512 a reasonable requirement?
Now that both versions work are their any major advantages to use Pro vs X3c other than Breverb?

For Exporting it doesn't matter a bit how high they are - raise them as high as you need so that you get a usable result.