Cheers for the reply so far guys, I realised the speedo would be out but someone mentioned I could get it recalibrate? Or is that just more hassle than it's worth?
Also, I've had a look at the offset and I think it only pushes the rims out by an extra 20mm which shouldn't be too much of an issue I don't think? My major concern is the arch gap, where I lose over an inch of clearance, and I have no idea how to determine the maximum play in the suspension. Is there a way to figure it out, short of fitting the new wheels and pressing down on the corners? Last thing I want to do is hit a bump and ram the wheels into the arch.