I think the only time that the wattage load should make a difference is when the radiator has peaked out. A radiator will only be able to dissipate so much heat, and when you have reached that limit you will start to see stuff like run-away systems, but until then it should be the relatively the same thing.

Again, with the mdot*Cp*(T2-T1)

knowing the wattage, T1 and the flowrate you can solve for T2. That will be neglecting natural convection and radiation losses, but without and fans those two will be small. You can graph that equation for a constant Q and see how T2 will vary with your mdot if T1 is constant.

It would be nice to see a comparison of the equation versus data if you are running a few tests without any fans. That will give you an idea of losses from the radiator itself.