Ah yes, I missed that "clock for clock" statement, I guess I was too focused on the 1 - 3% figure. So if it's 5% clocked slower, and 1-3% slower still clock for clock, how slow would that make it? How does that work exactly... around 7%?
Unlike CPU's, a 5% reduction in the speed of a GPU does not mean 5% lower performance as there are more factors in the big picture than just the GPU frequency. If you are indeed talking about one of the downclocked cards then the difference would probably be 5% or so.
There are cards like the XFX card I own which has the same 600/1500/900 of a reference 9800GT and as such would only be 1 - 3% slower than a regular 9800GT.
You could always do a perfectly safe OverClock like 625/1562/925 Core/Shader/Memory which should negate the performance loss the EE cards seem to suffer.
I'm so CPU limited by my stock clocked 4200+ x2 939 CPU that I'm not even remotely worried about OverClocking my card although I did test it at 715/1788/1025 without Artifacts which goes to show that EE or not some of these cards have the potential to best even the fastest production 9800GT's.