I just went through the trouble of replacing the stock Corsair H55 fan with an SP120 and remounting the pump with AS5, but temperatures have not changed a bit. Hell, they've gone up. Ambient hovers around 28ºC, CPU idles at 36ºC, goes to around 85ºC after two minutes of prime95. I also noticed that the pump/heatsink of the H55 gets rather warm, but the rad stays cool no matter what. I think the pump may be having some troubles, either that or I didn't apply the AS5 correctly. (Took me about 6 tries to get it on)
There are two situations where this scenario will happen. Either A. You need to replace the pump due to it not moving the hot coolant to the radiator, or B. You could try switching your coolant.