It is possible to go up a little but because of the shaft drive the only way to get the bigger tyre on is with aftermarket stuff. The kicker being that with a 180 rear tyre unless you go really crazy and modify the rear to chain drive the wheel has to be offset for the tyre to clear the shaft.
This means that the rear wheel is out of line with the front. On a drag strip running quarters that's unimportant, turning right on the street is altogether more entertaining.
Next time I see my friend who is vmax nuts, he has retired and buys mot fails and abandoned projects and rebuilds them so they go back on the road. Nothing flash just legal and mot'd. fag money he calls it. I will ask him, he walks past the house 2 or 3 times a week.
On a personal note, I nearly bought one of his creations. DEchromed with straight bars and a mild tune with big heads and a wonderfully evil crackly sounding pipe that was glorious on boost, shite off!. Stock wheels etc with I think some uprated (relative) calipers. They say not to ride your teen fantasy's but I knew they were quirky and was not disappointed, it was oooozing ' I know I'm different, loud and if you don't like it fuck off', was really hard not buying that bike. Things don't get me like that very often.
His personal vmax is a candy purple nitrous thing with mikuni's forged this and that and 190 odd bhp at the wheel on gas (when the stars align and it actually runs properly). That has the huge rear wheel and if you ask me it looks daft. A vmax is all about the engine, handling is bad enough as it is why make it worse. Spend the money on the kit and know how to balance the carbs and set up the vboost properly that is far more important than a back tyre.