Sorry I am late... I was in the kitchen.
I love my 300! For me I have no desires to upgrade for a couple more years...if ever. The Brock's system really woke her up and I have no problems keeping up on rides.
I am not very experienced on The track with her and I have not experienced her full potential either...doubt I ever will.
The KLX250SF I have ran the shit out of on the cart track. But again still have not experienced the full potential.
I only traded the 250 for the 300 because I wanted a FI bike and a little more power (potential).
I test rode the KTM 390 duke....it was a blast.