More power generates more heat, you may just have exceeded your systems capacity.
Another simple possibility is that you have too much coolant in the system and it is just venting the excess. A lot of folks fill the system up and wait for the thermostat to open and fill it some more, which isn't wrong, but if your system is open and circulating and full, the coolant will expand as it heats and have to go somewhere.
With the engine running and the radiator open let the care reach the point that the thermostat opens. fill to 2 -3 inches below the top of the radiator core. That is enough coolant, more and it will overflow.
Change to a 16 pound cap, it raises the boiling point and will help reduce any overheating issues.
If those simple and cheap measures don't get you back to a properly performing cooling system, then either upgrade the water pump to a high volume pump, or upgrade to a larger aluminum radiator or better yet do both.