Witnessing the 'the Burner' blitzing a class field aboard a trick XR400 at a Picton Thumpernats round back in the pre CR450F days was one of the more memorable rides I've ever seen. He was pulling away from from YZ400F, Husaberg and KTM mounted opposition on the long downhill straight, showing everyone that you could find some serious HP in the little XR engine if you knew what you were doing. That bike and Glenn Bell's XR600, which was also a very fast bit of gear could easily be replicated with todays eBay parts sourcing and ever growing XR speed equipment industry thanks to our American flat track brothers.
I'd still rather a Rotax powered Kato or even better an ATK because of my love of the unusual, plus of course the fact that the Rotax engine is just so bloody good.