So what happened to the diff clock driver, did you ever get it to work as expected?
Yes, it's not a perfect square but it works nicely.
I've tested many different drive strengths and termination resistors which didn't have much effect on overclocking.
There seems to be a wide margin.
The included files are set to LVDS 2.5V for the synth, which is the recommended setting
For extreme overclockers you may get a little more speed using SSTL 3.3 as it's even more square and faster rise/fall time.
However with SSTL the signal swing is so large it's way out of spec and not recommended.