nope..i use my i7 2600k at 5.3 ghz for benching with HT. its stable. so plz clear ur misconception first. also most 2600k can OC slightly higher than 2500k. Howver for gaming only, you are better off with 2500k or 2550k
Regarding performance improvement with a 2500k over q9450 @ 4ghz using a HD...