[Yellow Beast]

I'm looking to put 15mm or 20mm wheel spacers on the rear of my 2017 1ss. I have aftermarket wheels on the rear. 20X11 5X120 43ET WITH
305/30/20 TIRES. I want the wheels sticking out past the fenders a little to get a wider stance. Should I anticipate any issues? Fitment, rub,load on the wheel bearings.

[Msquared]

Assuming these are true "plain" spacers (and I think 15mm cannot come as an adapter), the first concern is you'll need longer wheel studs. Aside from that, yes there might be a concern of the tire rubbing at the top of the fender opening if the suspension goes to full compression. Your rear wheels and tires are identical to SS 1LE dimensions, but I'm not sure how far you can stick them out of the fenders without rubbing at the top. Also, the answer to that question will depend on how much negative camber you dial in: the more negative camber, the further you can push them out of the fender opening.