Bluevelvet, count your blessings that the dealer replaced the head gaskets for you !
I just did the head gaskets, cam seals and timing belt kit on my 09 Impreza and it was the job from hell.Had to cut out the valve covers due to all the shouldered bolts being seized or snapping off.
As for your loss of power , your timing belt might be off a tooth or so,it's pretty easy to mess up if you're not careful when installing it !