BTW, to answer the OP's question, 15's WILL fit, but not all 15's. YOu will need to research each model. Backspacing and such is important but the design and shape of the inside of the wheel is really what you need to look at. For example, here is one situation MichaelW had where the rim deminsions fit but the design of the back plate didn't:
CLICK HERE
Most likely, you will end up needing to run steel wheels to fit 15's on with the size of the front calipers because the material is thinner then say a forged or composite wheel.