I've considered the inner arch too in my post; that I didn't mention the strut, was due to me knowing, from experience on my EP3, that (in all liklihood with the DC5 too) you'd have to be fouling the inner arch before you foul the strut...
And yes... it's not set in stone... I know someone running 9" wide +20's on an EP3, so high offsets aren't mandatory for wide wheels, but to keep a Type-R functional - the way a Type-R's meant to be driven (by most peoples accounts), the wider the wheels are, it is advisable that the offset should be higher to pull the width off - but it's not a given..
My Mugen's, Mugen-specified specific to the DC5, are 8" wide and they are +55. I had Work Emotion's 7" +42 and they sat very similarly to the way my Mugens sit; there's probably a few mm in it, but it's very similar indeed... that's why offset, IMO, is very much relative/dependant on/to the width of the wheel.. had my Mugens on an 8" wide been +42 they wouldn't sit the way the Work Emotion's did...
Hope that clarifies, if my first post wasn't thorough enough?
You've not said anything I haven't - just elaborated that is all.