Heavy rims, wider/widest tires available, tallest rim sizes, the F4 adjustable Aero kits and the Limited Slip Differentials all add weight and drag and lower the number. Converting to 2WD lowers it also in most cases, depends on how much weight you lose vs how much launch/acceleration difference.
However... Adding weight and decreasing power are counter intuitive to my mind.
I can understand why you'd want to do this, but there's more to be gained adding a tunable race trans, race brakes, race suspension, clutch and flywheel and tire compound/tire width to a lower car. Adding the F4 Aero helps too, and the few points it gives back may net you a little hp boost. Having "extra" hp is useless unless you make it useful.
I'd much rather be able to control shift points, add Aero to create down force and be able to adjust brake bias and pressure and chassis settings to use the often more than adequate hp present.
There are some basic tuning changes you can make that will help anyone better use a car that are pretty flexible and cure 90% of what plagues stock setups, basic fundamental type things that can be used on just about any car and then fine tuned from there. If anyone isn't comfortable with tuning a chassis, I can walk through some of them in a separate post.