I am not 100% sure, but I did some math for you that may help.
For 17x& Pegasus wheels with offset 46, the distance from the hub to the outer edge of the wheel is 55.6mm
For a wheel that is 17x7.5 with offset 49, the outer edge is only 46.3mm from the hub. This is ~10mm closer. Pegasus wheels also have very flat spokes which maximize caliper clearance. While 10mm spacers may work, 15mm may be a safer bet. It all depends upon the spoke design.
Thanks guys! My project was to put bigger spacers at rear wheels for aesthetic. That way, I'll have to go with 20 mm at rear. Anybody tried 20 mm spacers? Problems with rear fenders or even rear hub bearings?