I don’t know if you can fit a wider tire on a stock rim the way you can a car, so this might be moot. But…
On a bike, tire profile has a very strong impact on the handling of the bike. Rounder, more pointed, flatter centre for touring… these all affect tip in, road holding, and straight line stability.
Side wall profile and flex affects these as well.
Side wall flex is a rather disturbing thing to deal with (I recently forgot to check my tire pressure and the back end was “sliding” down the pavement and moving around on me… I was only down 2 psi, but it gave the sidewalls enough flex to really disrupt my riding).
Less sidewall (like sport tires) means less flex, better cornering control, but less comfort on the road because the sidewall absorbes bumps (hence why the back tires on cruisers usually have a big sidewall).
Then you have issues of clearance. Lots of fenders come real close to the tires, and at speed, tires flex, getting taller.
There are a tonne of issue to worry about when changing tire sizes. In a car, those changes are less pronounced because, well, you have four tires and you only ever roll on one part of them.
What I’m trying to say is that altering your tire size can be complicated. I have not seen anyone on this forum talk like an experienced customization mechanic. Similarly, I’ve seen lots of squids on other forums sound off like they know everything, but are not people I’d trust with the life of my cactus, let alone my own life.
You should chat with someone at a shop that specializes in motorcycle customization close to your style.
If you are pimping a cruiser, find a cruiser shop. If you are pimping a street bike (you went with a GS500 right?), find a shop that does track conversions.
One place local to me is Hybrid Racing. http://www.hybridracing.ca/Home.asp
They specialize in customizing sport bikes and building track-day and drag racing bikes. The guys on my local forum speak very highly of them, in particular when it comes tires. They know their brands and know how different tire brands and models work on different bikes. Drop them an e-mail; asking experts is always worth your time. No sense in wasting time and money when you can do it right the first time…
Just my 2 cents