The power case makes the phone much larger and heavier. It also means the standard cases don't fit.
If you have a S2 without the power case this mount will be way to big - scaling it down a little may work but that is an exercise for the reader.
There seem to be more than one model of case so YMMV. What I have is called "Samsung Galaxy SII 1600mAh Power Case".
This started as a belt mount but I've done a wall mount first.