Well, you have two groups of people; one who wishes their 250F had more grunt, and one who wish their 450 was as nimble as their 250F. What they want, then, is a bike as light and agile as the 250F, but with power like a 450, and unfortunately, nothing like that exists, nor, frankly, is it likely to.
I rode a KTM 350 a while ago, wasn't impressed, really. I think the suspension kept me from enjoying the ride as much as I should have, but it was neither as nimble as a 250F, nor did it have power like a 450. It was definitely fast, but lacked the low end torque a 450 has.