Welcome Dima!
Leaving it stock is a valid plan, but lets rephrase your question to something we can actually answer. If you find putting around stock "boring", then is it safe to assume you want a bit more pep? Are you willing to accept a little less mileage for a performance level that make you happy? Should your question be? "Where is the happy middle between mileage and performance on the 300?"
Also, how hard you ride it will have a large effect on your mileage, but I am sure you know that. Many 250 racers only get around 35mpg.
