You'll be fine with the 27's, with the proper mods a 500 can turn 28's pretty easy. But you can turn 27's easily stock. And you can go ahead and install the tires, they dont have to be installed at a certain time. But yeah the MST's from H/L are great tires and you will be very happy with them.

yfz 450 hands down..it's a quick lil bike