You need to learn to jet your bike mate. Even if you buy a 300 it will need to be jetted correctly to get the most out of it. As Glenn has indicated, the jetting can make the difference between your 250 feeling like a 300 and a 125.
My 250 will still lug off idle just as clean and torquey as my 300 did. Its the power below the band thats not as strong, and then once on the pipe things are a bit more similar. The 250 does feel more aggressive, or that it needs to be ridden more aggressively to go as fast. It sounds like you'd like the characteristics of the 300 for what you do though. You can get a kit from gofasters which will set you back around $800 USD plus fluids and gaskets, then labour if you're going to have someone else do the work.
Jetting your bike yourself is a reasonably cheap exercise. A new tyre will cost you more, and the jetting will make your bike much more ridable than a new tyre will.
For head mods -
http://www.gasgasrider.org/html/measuring_squish.html