I'm using the cheap SPC V2 control arms for autocross, track use, and daily driving. Plenty of other track guys are using them too because they're cheap and they hold up well. No concerns there, IMO.
If you're going to 18" you can't fit a 255/40, it'll be way too tall. You'd have to drop to a 255/35. That narrows your choices a little
See what I mean here:
http://www.willtheyfit.com/index.php...culate#content
I'd play around with the fitment choices on the willtheyfit site, it's helpful for visualizing the changes. Ignore the strut picture shown, it's not for a BRZ/FR-S.