I don't think speed is the issue with Hp. Less Hp will bog down under a load where more Hp will keep on spinning.
Many tools run at the same speeds but come in lots of different Hp.
I think the thing you will see with a higher HP machine is you can apply more presure and do larger areas than with the lower Hp ones.
A 1/3hp you can only run one wheel, but a 2hp you should be able to stack 3 wheels or so and do a wider area at a time.
Also with the higher Hp if you want you can apply more presure (with in reason) to the part and cut it faster at the same motor speed. Of course with more presure you will most likely get more heat. When I used to polish range handles for plating at a chroming company I would apply fairly hard presure to cut the orange peel and get into tight spots then a light pressure to buff the handle to a high shine. If we would have had a Low Hp buffer you'd be there all day
Chromo