I had a 53/39 set up and changed it for 50/34 for a trip up the alps a couple of months ago; I don't think its changed my speed except possibly I'm spinning more. So IMO a 50t is big enough, in fact my top speed this year was (45.6mph) was set on a borrowed cyclocross with a 48t, and have regularly broke the 40mph barrier with the 50t, in theory you can lose out on maximum speed but that doesnt seem to be the case for me.