I did 2 years and 20K miles on an '02 GS500. Lots of people track em. They don't make big power, but they're big on reliability. There's all sorts of little mods you can do to make em more trackworthy. You can add a katana rear to get it up to 4.5" from the stock 3.5", which lets you run better rubber (although the pirelli sport demons are available in a 130/17/70 and work amazing for the bike), you can get sonic springs for the front, or swap front ends fairly easily for double disc setups, katana shocks swap in for better adjustability/damping, and they change the rake a bit for quicker turn in. I've never had a problem with my transmission, it felt sharp, just like my SV. You can get case guards for the nakeds that'll stand up to just about anything you can throw at em (and double as highway pegs, to boot). You can add clipons if you get sick of the standard bars, or add Suburban Machinery bars. Changing the gearing lets you get more snap off the line, as long as you don't mine hovering around redline all the time (and as a tracker, you shouldn't mind). Seating position may not be comfortable if you're over 5'11 or so, I'm 5'3, and it was fine for me. You can chop the subframe to lose weight. Keep the oil changed in em, and they'll run over 100K miles no problem. Parts are plentiful as they haven't changed mechanically from '89 to '02, and in '04 the only mechanical changes were a switch to an electrical tach instead of cable driven, and the addition of an oil cooler.
that's all I can think of off the top of my head. As you can tell, I'm something of an enthusiast.