Wow, i'm impressed with how you were able to mask out the hair! that is usually the toughest thing to do when dealing with pasting people over background. I'm sure his technique can be also done in Photoshop, so im keeping it for reference, cause i'll need it!
Yes, in Photoshop this will go smoother, I'm sure! The filters and tools are all the same, only I've noticed that GIMP takes longer to calculate and still misses some features I've gotten used to in Photoshop, like a grey-scale representation of the RGB channels; it's often easier to start the mask with one of those ;-)