The general consensus is that since C2D IHS's are bowed, a bowed water block will have better contact and thus lower temps. I've seen a few reports that a bowed GTX is better than flat, etc. and I would have to assume that this is true since both the Apogee GTX and Fusion are bowed by Swiftech and D-Tek, respectively. If you purchased your block(s) recently, (like, after July) they are bowed.