[economix]

If you trim that area, under hard breaking (track day or say, an emergency), you risk blowing the backing plate off of the pistons. That high hump area is there for rigidity and to reduce flexing during the piston transfer, if you thin that out, the caliper will crack and it will be during a time that you may not have a chance to regret.

Please be safe, be smart, and get spacers or fit the wheel proper. Do not modify the caliper the way you're asking to.