So I've gone up one to 16 teeth now. Thoroughly recommend this mod. Much more relaxed coming up through the gears and the bike will happily chunter all day along the green lanes in 2nd at up 20 mph, just like my old beloved 350 classic trials used to. As previously reported, 70 mph now a reality at 400 or 500 rpm lower than previously. Should be standard in my view. No noticeable adverse effect on performance. My local RE dealer fitted the sprocket ( from our host) for me in half an hour for 25 quid.