I went from stock to road 5s front and back with the 55 profile. I'm not really sure how much it is with it just being a better tire, as I didn't really like the OEM, but I feel like it is a bit more crisp with turn in, and a lot more confidence inspiring.
I'm not sure I'll even bother going back to a 50 to find out, I've been very happy with the 55 so far, so personally I'll just stick with what's close to it, spec sheet wise and go from there.
I am curious though, if someone has ran the same tire 50 and 55 and felt a difference? I can't see it being much, as I think the general quality of the tire makes the most difference. (Screw the OEMs lol)