I have used them both.
The Power IMO is a greatly improved tire. The HPX hit the market designed for the hyper bikes, Busa and the 12. It does provide good stick and decent mileage when pushed hard.
I got 1500 miles or so out of the front and rear.
The Power has more of a "V" shaped profile and will make your bike steer a little quicker. If you are used to a true race tire 208GP, Pilot H2/M2/S2, the transition will not be noticable.
If you are coming from a round shaped tire, M1, Diablo, 208, BT56/10 you will notice a difference.
I do not do any top speed runs but on the back straight at RA, I was seeing right at 180 indicated with no tire problems using the Michelin M2 120 ft 190 rear.
I used the Powers on the street at last years Busa Bash. Excellent stick, better than the HPX and no warm up time. After a few corners on the way to the Gap, I felt confident and ready to rail.
I got around 1700 miles out of that set and I was pushing harder than I did with the HPX.
If you are looking for a track day tire, I am using the Michelin M2. Awsome stick and the once warm will help elimanate cold tearing of the tire due to improved stick over a street tire.
I actually get better mileage out of the M2's around 2200 miles.