well mark its like this..
water pulls heat out of you computer faster...
Then the heat gets outside faster.
People often think that watercooling will make your room cooler, but in truth, its the oposite. Your room gets hotter without circulation.
If you have enough air going though your room, 100F temperatures is 38C Bottom window coolant temps.
If your system is good, that means your coolant will be ~ 40-41C, with a computer idle of around 47-50C.
Load will push your temps to around coolant temps 45-47C, which then we approximate.. that would give you roughtly 77-80C.
^ that looks like air temps on a normal day to me.
As i said its in your system that you plan, a 200 dollar h2o setup could get close to that, you would however need a 250 dollar setup for my rule above to apply.
Is 250 dollars worth it to you? That is the question you need to ask.
But no i never downclock any of my systems because i dont see the need to. None of them are anywhere near the 80C warning mark. Infact ive never broken 80C on any i7 b4.
This is where water does better then air. Your milking the efficiency of water when your ambients are higher, or you have a higher heat load.