I have seen people have fun on all sorts of bikes on the track. Our current track day provider just created a Ninja 300 mock race series that a bunch of people have purchased new/wrecked bikes for. These are people that have Ducati's, Aprilia's, BMW's, etc.
A few points though. The size of the track you plan to go and who you ride with will change that experience. A tight, technical track will be the bomb and a track like Road America could leave you wanting more HP. If the people you go with all have bigger bikes, unless the track is technical or your skills are better, you won't be riding with them much. If you are just doing this for yourself to improve some skills, have at it with any bike.
A few points though. The size of the track you plan to go and who you ride with will change that experience. A tight, technical track will be the bomb and a track like Road America could leave you wanting more HP. If the people you go with all have bigger bikes, unless the track is technical or your skills are better, you won't be riding with them much. If you are just doing this for yourself to improve some skills, have at it with any bike.