Dunno about the offset, but when my valver went kaboom, i put my 17s on my old RT. It has valver front arches with the inner arches taken out.
I was quite shocked to find they went on the car very easily, they only scrubb slightly on full lock and when goin over some harsher bumps. My RTs dropped 60mm too, although RTs start life way higher than a valver so its not that low.
Oh and the valver front wings are about £35 each direct from renault and are quite easy to fit once the inner arch has come out.
Its amazing how small the phase II wheels look now after getting used to 17s!
Warning though, handling suffers big time! Proper tramlines!