The E30 fitment BBS RS typically came in 15x7 and 16x7 ET25 from what I recall, so widening them at all pushes the offset lower. Anything more than .5" of extra width on them basically means going past the optimal point for tire clearance, and really becomes an aesthetic mod rather than performance.
For example a wheel with a 1" lip (becoming 7.5" wide overall) would become ET19, and that is about the limit to fit nice wide tires. An 8" wheel becomes ET13, and you're pretty much stuck with stretched tires at that point. Of course it's all personal preference, I'm just pointing out what you can and can't work with.
I don't know what offset the G60 RS' are, find that out and I can help you.