The 06 600RR is a highly refined 03. The 07-presant are the newest generation and they are vastly improved over the old model. Yes, it is a fantastic bike. I'd say awsome, but its getting long in the tooth now.
I vote 450 dirt bike with an extra set of wheels. One set knobbies, another set street tires. Don't have to get 17s unless you just have to have the coolest gear. The 21/18 will work great on a kart track also. Again, assuming you live near tight roads, or a kart track. In this senario you have two bikes in one. Either of which are the type of bike that don't get ridden much. So why have two collecting dust 90% of the time when you can have one collecting dust 80% of the time.
If you just have the need for speed and can't do an SM bike....then I cant make a suggestion. There is just too many to pic from and too many opinions about which is best. Any super sport will do. Be it a 600 or a 1000. Or 1200 twin. Or 1050/675 tripple. 850 twin....You get the point.
02 919 (RIP)