I'm going to stay with the 525 series. I don't plan to travel outside the US with the bike, and my riding venues will not get any more technically strenuous than what I'm doing now. Somewhere over the past three years I read of a 13-t counter-sprocket but I don't find it on the vendor sites I patronize. Maybe someone else will chime in with a supplier. Anyway, I agree the 14/45 and 16/45 seem to be the best pairing. On the low end a 14/45 puts it where a 13/42 would be, and on the top side a 16/45 is almost the same as the stock 15/42. Very likely it's where I'll head with it when I refresh the chain and sprockets.