The wheel catching with 20.5mm is wrong unless you go extremely low (over 40mm). I've got bilstein B12 suspension and 20.5mm spacers with absolutely no issues.

That is with the standard 205/40/17 wheels. I have managed to get them to rub slightly once (for a tiny split second) when I was doing 95+mph down a fairly bumpy country road trying to keep up with my friend's 350z (private road of course ). This was exceptional circumstances though and I don't expect to do it again anytime soon.

One thing to note, if you feel behind where the bumper joins the arch edge, there is a pretty sharp spike that sticks towards a tyres slightly. Well worth making sure you're not going to rub therefore.

I would say go for the 20.5mm spacers. They set the rear end off great and are no more difficult to fit.

I imagine the reason Goodyear's rub is something to do with the standard suspension in his car. The ST is firmer.