I don't see any noticable difference in games like GTA IV or some other Windows applications with HT on or off, but I think it will have an effect on video transcoding but I do not do video transcoding very often. The only noticable difference I see is that I have 2-3 degrees lower temps when HT is off.
Would my core i7 930 @ 2.8 Ghz with HT off be just as fast as an Intel core 2 Quad @ 2.8 Ghz?
Keep HT on, this just increases the efficiency of your cpu, 2-3 degrees is no reason to worry about.
I keep all powersaving features off, as well as turbo boost. I live in a retaively warm country, right now it's winter, ambient is around 20 degrees and my cpu idles 45 degrees, peaks at 80 degrees under heavy load.