your changing the gearing... so you lose acceleration and gain top speed, put tiny wheels on it and you would accelerate quicker and have a lower top speed. thats why your meant to keep the rolling radius the same.
yes, the weight and width of the wheels will create drag, and the heavier the wheel the harder the car works to spin it, but they aint going to be heavier than the phase 2s if thats what you have.
you arent losing BHP... thats at the engine end, just changing your gear ratios....