Taking the blades off will not change the speed of your mower. That's a safety issue, no more, no less.
Bigger pulley on the front, smaller on the rear equals faster. However, before you go getting crazy with speed, think about the steering. How tight is it? Is it all nice and sloppy, as most are, so the first time you break 10MPH you loose control, flip over, and break your neck? Don't laugh, it very well could happen.
Are you going to open up the transmission/transaxle and make reliability modifications? If not, how long will that rascal last at anything over 10MPH?
Are you going to lower or widen the mower for stability?
My suggestion would be to go in steps, learn to trot before you go for a full on gallop. Put a pulley on the back half as big as what's there now, see what happens. Get it sorted out, then put a larger one on the front.
I think you'll be more successful in going fast AND staying healthy. Being all the happier for it.
Dale