The seat may be sprung and no air?
The biggest limit to maintaining momentum on our Mog with 200ish hp and at 9500kg was the big gap between 7th and 8th. If I had bought an overdrive it would have been much better (I assume) to be able to have seven and a half. But going from our previous slightly less hp mog to that was much better. The folks saying 170hp is ok have a very different transmission and ratios to our Mog, so hp and overall ratios may not be the only issue.
From a racing point of view surely adding hp would be secondary to saving weight? Same here I am sure. No hottubs!
Bill Caid went from a Mog to 1017A and put super singles on the back (at 20"). I think they were U500 rims on backwards?
http://www.billcaid.com/index.html