Nah... it's not that... you seem hung up on top end speed. Why?

It's not like you said the 500 looked better than the 250... the diff in top speed is only 10mph, and with either bike your are going to find out in a hurry how much faster 600's are. Considering that 10mph on top is a non issue.

If you are looking for something to grow on I'd say try an SV650, tons of aftermarket parts for it.