Wheel spacers will effectively change your wheel offsets, but they won't have any effect on the amount of tire you can mount on that wheel. If you go that way, be sure that the new spacers have hub centering rings, and make sure you use new, quality studs. ARP have a racing stud that is longer than the OEM and should work OK with spacers. If you want to run closed end nuts you may have to trim the end of the studs a bit. Also be aware that the more outboard you move your wheels/tires, the more load you're putting on the wheel bearings, so they'll need changing more often.
FWIW, a number of us run 15mm spacers on our 550 Maranellos, and they work fine, and do improve the handling as well as the "stance" of the car. H&R make a quality product, as do Hill Engineering (however Hill's are only for Ferrari's)