The aspect ratio of a tyre is the tyre's height or profile expressed as a percentage of the width. If my maths is correct a 190/50 has a 95mm profile & a 190/55 is 104.5.
I changed from a 50 to a 55 on a GSX-R 1000 k6. I felt turn in was slightly quicker as a result of the taller profile but that sensation quickly went away after a few miles. That suggests, to me, that it's quite subtle & is a track/racing driven decision by tyre companies. IMHO. Great forum.