I have geforce cards(standard one) in my mac pro.
I have noticed a definite lag in the single link output,
Compared to the dual link , When running the same media.
One output is a few frames behind.
This is especially noticable when doing quick bursts.
It is almost impossible to sync the 2 outputs.

I have decided to order the ATI Radeon that will give me 2 x Dual Link outputs.