It's not a difference between 1 speed and 2 speed, as demonstrated by the Myron's chart.
The main difference I can tell is the gearing (14t front vs 16t front for 1 speed) and the exhaust (1 hole vs 4 holes.) Everything else appears to be the same between 1.5hp and 2hp. The missing information on both of these charts is the amount of restriction in the air box. My hunch is that the 1.5hp model had less air box restriction but more exhaust restriction than the 2hp version.