power to weight ratio's are more important in motorcycles (or just look at performance figures), unless you are just using Motorcycles to extend your penis, you can have fun in any sportsbike regardless of its power... and doesn't a CBR weigh alot more compared to the Ninja? and incase you're wondering, I have a friend who rides a Ninja, and let me tell you, it isn't considered slow at all.
Ninja is probably a go...
unless i quickly discovers how to ride during the moto learner class/course
then its definately 600cc's