Yes heat rises, but put a simple fan in a computer case and hot air will blow where ever the fan is pushing it. If you had a case with no fans and could only pick one side to place holes then yes the top would be ideal. But once you set up airflow the heat rising doesn't really matter.
To answer the OP. Yes you can reverse it to intake cool ambient air thus cooling your CPU better. And yes this will push the now created hot air into the case. That's the trade off for having lower temps on those cpu water loops. Typically isn't to big of a deal if you can exhaust the hot air efficiently. Which in your case might not be easy. The hot air will be pushed up and then back to the graphics card ultimately leading to higher GPU temps. Question is how hot is to hot for you? All depends on the temp and amount of hot air coming off that radiator.
Best bet is to test it. Do a bunch of idle and load test with your current set up and record your temps (room temp too). Then turn the fans around on the radiator and test again. See if the difference is worth it to you.
Keep in mind too if you do this you may want to re-think your airflow situation. If you end up keeping *4 fans on the radiator as intake then I'd suggest flipping the top fans to exhaust. Not because hot air rises, but because it will give a better direction of airflow. Bottom front intaking air and being pushed in the general direction of top/back, then the top/back fans will help facilitate this hopefully removing the hot air quicker.
PHhewww..lots of typing. Hope I helped. I found out this sort of thing mostly comes down to testing various configurations to find out what works best for you and your components.