Tristan18au,
I have an '03 KX250 also and it is not a top-end screamer. Short of porting I doubt you will be able to get a whole lot more on top. I added an FMF Factory Fatty pipe and PV spacer and a VForce reed cage to mine and it works alot better than stock. The hesitation off idle is gone, the midrange is stronger, and it adds a little on top too.
As far as jetting, currently I'm running one size leaner on the pilot and main. But I'm running race gas at 1,500ft(~450m) elev. and ~70%humidity, so these settings may not work for you.
Leaning your bike out a bit could help with the hesitation off idle, also, switching to race gas or 50/50 race/pump may help make the bike SLIGHTY "snappier". The VForce helps out on the low-end as does the PV spacer, and the pipe adds a little everywhere. If you prefer more top-end, I've talked to several people who really like the Pro Circuit pipe for more power on top, but some say that they lost a little low-end.
Hope this helps a little.