Great to see you on OcuK Dave. I assume you're the AMD dave from beyond3d.
It makes both gpu's work evenly with 99% usage. According to my testing this sometimes adds a few extra percent performance. For example with ULPS on gpu1 works at 100%, gpu2 works at 75-95%. It varies. With ULPS off and assuming no cpu bottlenecks are in place you get equal gpu usage of 99% per gpu. This often shows itself as a small performance increase and a very slight, often unnoticeable (certainly since the arrival of frame pacing) smoothness increase. Overclocking seems to exacerbate the problem even further. At stock its not such of an issue, it seems.
A few of us
here did some testing. This was a while back now but id be happy to draw up some solid figures regarding the performance difference in fps terms between ULPS on and ULPS off stock and overclocked.
EDIT
I should add this was before frame pacing arrived. That did change things a bit. However you still get a small boost to performance with ULPS off.
I noticed that using GPU-Z also sends one of my 7950's into gpu 99% usage Dave. Seem to be a bug on the recent drivers. Obviously its affecting 290X users in a more serious way though with the screen flickering.