I just put a factory unit into my MY10 STI. Only took about 90min and all went well. Do you have the instructions that came with the unit? They are not bad but a couple of points in them need modification slightly IMHO... also, I did it without unbolting the seat (as that looked way too hard!).
So let me know if you have the instructions and I'll just tell you my tips. Otherwise I'll try to post or link to the instructions as well.
i feel like a sub under my ass would get annoying, and i think you will too. what i'm going to do is run all my wires for the amp to my trunk and hooking up 2 10" punches. you could just buy a box and do the same it will have to same effect just not as intense... i have an '05 wrx and the stock door speakers suck so i have some new ones on the way to balance out the bass.