It not just how much do you want to spend, I'm sure Steve can get 60+ out of any fresh '18 or 21' with just a few jets, a pair of wire cutters, a soldering iron and a holesaw. It's how close to the edge do you want to jet it? I saw 4 bhp from one jet change and then probably a couple more from going down another size and putting the lid back on the airbox and it was absolutely flying and definately on the lean side, then it seized

Mid 50's (which is what the UK "full power" 250's really produced) is probably a safer number that the mid 60's we all tend to chase.

Chambers are worth a hp or 2 peak and some overrev but cost 5+ midrange so they're wasted on the road, Andy and co are trying to find some compromise there (see here), a couple of holes in the lid is better than the "proper" modded airbox, the jet kit is more to allow tweaking, you don't need it all (although the needles are nice) but you do need the jets to hand it to sort the jetting out in the first place, especially if you're chasing either hp or a nice crisp throttle respose.
And intake mods are very subjective, as RC46 found, his dyno man didn't like the carbon ones at all, I've got some Boyseen reeds, some (RC46's) carbon Tyga ones and will pick up some HRC rectifers from Andy this weekend I hope and come the nice weather I'll be seeing whats what...this is when you really need the jet kit
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Please do not PM me technical questions, if you can't find it on the Forum start a thread