14t

I put the 14t on my 620 and it made a world of difference. You shouldn't get your hopes up too much about increasing the amount of power in that motor though. I mean, you can probably only realistically get 6 to 10 horsepower extra total out of any and all mods you do, other than increasing displacement.

Now, what you can focus on is how that power is delivered and how it compliments the handling of that bike. Try and hsave off as much weight as possible, and maybe get a lighter flywheel and you'll have quicker launches from a start and out of a corner. Going up 2 teeth in the rear ( I asuume this is what you mean. Going down would negate the change to the front) would make the gearing incredibly short and probably only suitable if you want to just tool around town, or race on a very short track. I think your freeway RPM at 80 mph indicated would increase to about 6500 RPM. That's around the torque peak, so it'll be responsive at that speed, but it'll be buzzy as hell and you'll have to maintain more throttle input to maintain speed.

As for getting your exhaust on one side, you can always look into doing a 2-into-1 system. There's another thread on that topic that was posted recently. The options are endless.