Think BH5 is the chassis code... Since it's a 2l I'm guessing it will be an EJ non turbo engine, not exactly a gutsy engine in NA form. Might be a good idea to check the condition of the air filter
It should be significantly louder, all depends on how much noise your backbox kills. With no resonator, it may drone along the motorway. Mine was very loud, but I did have a vortex afterburner back box which did little in noise reduction :D
You won't need a remap as long as you're not touching the downpipe. I did this mod and ran it for about a year with zero issues, as long as you get a cat pipe section which has a hole for the o2 sensor you should be fine.
K&N is oiled so has a chance of oiling up the MAF. I've used pipercross before and had no issues, remember the light side is the clean side, the rough black stuff on the bottom is the first part to get hit with the dirty air. Cosworth is also a very good choice
It should fit but you may have to modify the hangers if you need to use them... unless the system is light enough. I know the third hanger on the backbox points the wrong way, not sure about the centre pipe hanger
As in a full turbo back system? or cat back?
Turbo back would fit, but one or two hangers will be in the wrong position.
Cat back won't fit... centre pipes are different lengths
Something like this probably:
http://www.ebay.co.uk/itm/DFP-36-00-Subaru-Impreza-Single-Double-Din-Car-Stereo-Facia-Panel-Adaptor-/352097513027?epid=2099307653&hash=item51faa5ae43:g:qsQAAOSwPK1ZT-3R
Sounds like you've covered a lot of bases already! I would fire some redex into the tank... maybe for a few tanks, stuff is good for clearing gunk that builds up on injectors and fuel lines...
I take it the air filter is clean?