I had a chance to ride a new TM 300 MX the other day, and was very impressed with the engine. It had abundant torque with a very linear pull and fantastic throttle response. At one point, I slowed right down and shifted up a few gears just to see if it would pull it and it did, with surprising ease. If it was my bike, I would just gear it a bit taller to stretch out each gear a bit and you could ride it like a 450 - a very light and zippy 450.
Then I went to my own bike to actually race - a 2016 250 sx - thinking "wow, that was fantastic, I think I need a 300 kit" and ended up feeling really torn about it. No, I don't have that low end grunt to the extent of the TM 300, but my engine felt playful and fun in comparison (if that makes any sense?). The only KTM 300 I've ridden was an xc, and that engine felt almost boring - the TM felt a bit more aggressive, like you'd expect from an mx engine while the xc felt rather tame. Is the 300 sx kit much different than the 300xc? I'd hate to put a 300 kit on my bike only to find it boring. If it felt that that TM 300 I'd be happy (which is probably a useless comparison for most people since most people will never have ridden one). If it felt like the 300 xc I think I'd be disappointed. I hope this makes some sense - it can be difficult to articulate just what you felt when riding, and what you'd like.
Current rides: 2016 KTM 250SXF, 2017 KTM 350SXF