A PCs worst enemy, excluding heat, is dust. Dust is quite possibly the most aweful a PC can encounter. I once had a case, some of you have seen it in the Gallery section, and it had no fans, so my PC was very dusty, I took it to a LAN party one time and the owner of the house said, "Man that's a lot of dust, lets clean that out." So we went to the garage and used an air machine, blew sooooo much dust out of there you wouldn't believe! So from that experience alone I know how much dust can slow the CPU down because after all the dust was clear my CPU fan was running anywhere from 2 to 5 percent better, depending on the load it was under. So if you can have fans sucking the air in and then out you should be able to eliminate the dust factor by a large margin.
But here's where water cooling really shines. Since the waterblock is directly over the chip, you'll enoucnter no dust on that chip, and since your cooling device is enclosed inside a tube, that means your cooling devices won't get dust either. To me though, this is the only true benifit of getting water cooling, is the "no-dust" factor.