Water cooling isn't about low temperature, it's about being able to sink more heat. If you're moving so little air, the interior temperature starts to matter to the radiator, you need more fan. But once that water gets warm, the few degrees of difference from inside vs out isn't going to change much. Though yes, fresh air will get you a better low temperature point. But again you don't care about that, water is there for keeping the high temperature point down while under more strain.
And I don't care what Linus says, I know how hot/cold design works in a server room. I also build cooling systems to survive Phoenix Summer heat... I'll use whatever I can get. Because I never know when that air conditioner is going to cave! You can ignore the natural flow of heat, but you'll pay more in power to do it. Which in this case means, more fan... that's air flow you get for free, I'll take any CFM I can get, wherever I can get it.
But I suppose reinforcing convection isn't anywhere near as important as having a consistent flow. You could have air pumping in the top and out the front if you want. Just pick a direction and let it roll. I just usually intake on the front and side and exhaust the rear and back. Also, my tower is huge... radiator is inside the thing!
But beware, if you're cooling the radiator into the case, that's just moving heat to your other components.
