The SV650 is a great bike for the track. For about the same price that you would pay to replace the front end, you can build the stock one with new internals. Throw on a nice Penske, or Ohlins (or your choice brand) rear shock and it's ready to go. Of course expect to be passed on the straights by most of the folks on 600cc and up bike's. But if you leave your pride at home, and bring it out to have fun and build your skills, which is what trackdays are intended for, then the SV is a perfect bike. Parts and cheap, and you can pick up a used good condition SV for much cheaper than an equal year I4. You also won't chew through tire's as fast either! That's a plus in itself right there!
I had an SV trackbike briefly, but sold it because I could pass up on the deal for an 04 636.... Love the 636 on the track, but I think I really miss the sound of that SV more.