I ran a 70 all last season, then we started this season on a 60 - coz thats what was fitted from the showroom. Did Cadwell, Brands and Assen trackday on it and didn't really notice any difference (forgot it was a 60 actually).
However, I prefer a 70 - even it's psychological.
A racers head is as important, if not more important than anything else on the bike etc.