I should note that these post pertains to 96-02 4runner brakes that came with the 16" rim package. There are 4runners that had different smaller brakes and 15" rims as well(I have no info on those). The information wont exactly pertain to tacomas since there 15" and 16" were different calipers then 4runners.
I've been watching 15" tire prices for awhile and contemplating the switch. On average they are about 2/3rds the price of a 16" rim 35" tire. Not to mention this is a way into cheap steel bead locks. Initial investment is not any savings. Basicly I get free rims and don't save any money. But I also needed more backspacing anyways.
I bought procomp d window 3.75 bs 8" wide wheels. With a 1/4 spacer about 1/8" material is removed. I researched and someone else took much much more off to fit 15" alloys. Also I barely went deeper then two casting defects. Oh its scary, but I think I'm very safe