Just do it! my zx6r is 99 it came with 120-60 f & 170 - 60 b. I'm now running 120-70 f & 180-55 b. The 70 will not rub with 207rr,
pilot street, or pirelli race sc. Front end fills alot better lean over.
TAKE NOTE Tire size will change the way your bike handles.
Suspension adjustments for correct ride height.
IMHO the people at Kawi spent a lot of time figuring out the correct size of tires to put on the bike. I regret changing the rear size to 180 on my bike. I can't wait to wear the tire out of hit a nail to put a 170 back on it.
one local races with 70 ratio aspect, adn did pretty decent time. Since its a common fittment for most performance bike, shouldnt be a problem, but u may wanna filddle with the bike geometery alittle to offset for the increase in fork height