I have a GS250FW too. I have had the same probs with hard starting.
It all comes down to the idle jets (4 of them). As these engines are
quite small but have fairly big carbs the jets especially on idle have
to be tiny. Any gunk will block them in a matter of days. Make sure
once they are cleaned to have a filter installed. Also flush out your
tank. I can almost guarantee that there is rust in there.

As for no reving out all the way. The air box is a good suggestion
but it could also be the slides not moving freely. On my GS some kids
came around and toppled my bike while playing on it. This was enough
to bend the brass alignment rod that the slides move up on (if you
remove the top of the cab it has a brass protrusion). Just a small
bend of this will restrict the slide and no matter what throttle
application you give if the slide can't move you wont rev out.

cheers
Scott