Going fast in a line, isn't fun on any bike, I've gone pretty darn fast on my 600 and it's not really fun. Just wanted to do it to catch up to another rider who was blowin it just to see how fast my bike can go. I'll never do that again, honestly going 130 vs 230 (for example) doesn't feel any different.
Cornering on the other hand is just a beautiful thing! If you're timid in the corners on a 250, you'll only avoid them more when you get onto a 600. My bet is at first it will seem bulkier and of course, it is heavier which will scare you into thinking you'll drop it and can't hold it up in the corner if you need to. Learn to corner on your 250 with some confidence and then move up! Definitely watch Twist of the Wrist as well, some great points in there about cornering!
When the road is clear, I find the best way is to do the following:
-go at the right speed so you don't have to brake before entering your turn (helps overcome the mental block of braking before turns). It will feel too fast at first even if you're going at like 30 km/h
-WHEN THE COAST IS CLEAR, slowly start trying to do this at slightly higher speeds, try it at 34 km/h, 38, 40, etc. until you can stay well within your lane and you're at a higher speed.
-Avoid braking and taking a corner really slow with "steering"; rather turn with a slow "counter-steer"
-get some pylons/markers and practise U-turns and figure 8's in a parking lot
Don't upgrade to a 600 if you're not comfortable with a 250, it's only going to get worse and you'll be MORE scared to push your limits/learn to ride properly.