Nothing, it'll still say 250, just when its dressed up (eventually when I have the dosh) like a race rep it'll have a little better performance than a standard or mild tunned 250.
Also, it's cheaper to buy the kit (well it's looking that way) than to redo the top end on my NSR - its due soon (it's been 13000 kms since a top end rebuild).
I was thinking of tuning the 250, port flow etc, but then you have reliablity problems. The 300 kit looks like it offers 8 - 10 hp with relibilty, and just bolts on... Well i think it does, but so far my searching hasn't dragged anything up. Whether it's worth it or not I dunno.
I see StephenRC45 has had a few problems but is his kit a Tyga one?