I'm familiar with the springs. They are not helping your handling, quite the contrary. What they advertise as is a separate matter. Springs with that drop should have a much higher spring rate. The drop is about the same as Swift R springs. You can look up the rates, they're far higher. Would I recommend those for your car? No, they have no business being used with stock struts. The stock struts can't handle the tanabe springs you put on worth shit already let alone something with a much higher rate.
18x8.5 +42 is not that extreme. I would say you will drive around for a bit and think you're fine but with passengers or trunk weight you will rub on bumps, fixable with a rear fender roll. Either that, or get ~6mm more offset to ensure a clean fit.
pjkolosha, the wheel width you can run is dependent on the offset of the wheel. Low offset moves the wheel towards the fender as does adding width. It also matters what tire you want to run. Are you looking to run the widest wheel you can (I don't see why you would), the furthest out visually, or cram in the biggest tire you can. If you want to cram the biggest tire, get an 18x8.5" +48 wheel and put on some 255/35R18 tires. I'm not sure if grids come in that offset at that width though, fair warning if you're sold on that particular look.
BTW, I recommend 17" wheels and tires over 18" for our cars. More sidewall is a good thing. There is no performance reason to go 18" and it'll cost you a lot more money.
04 STi 16G powered lagless fun
04 WRX: SOLD