I am very skeptical at this moment about that horsepower result. Really, a 350 making more power than all the 450s? Seems fishy. And I test rode all the 2013 KTM MX bikes and the 350 in no way felt as powerful as the 450, even with the less weight.
I'll wait for the comparison tests to see.
As far as the 250SX goes, I could imagine they can get 55hp out of them by now. If they really cared. But there is no class to push them to have to do that.
There is a point on a MX track where there is too much power. Of course, that depends on the rider and their skill. Then the advantages of weight. As said, many pro 250 riders match the 450 lap times. I would personally probably be quickest on a 200, that is my theory. I can wring a 125/144 out but would like more bottom end. While in the desert, I can use all 55+hp of my 380 at times.