1. Not sure of the DIY costs. For the time being, the dealer is doing the maint.
2. Synthetic oil only. I believe the change interval is 3,750 miles. New filter every time.
3. No idea, only 4500 miles on mine
4. Cat back systems don't do much to increase flow. Anything that does increase flow will need to be tuned for with either an open source tuning set-up or Cobb AP. (Intake, downpipe...etc)
5. I have the short throw shifter, and while I can't say what the WRX is like without...I've driven other Subarus...and for dealer installed options....I'd get both the short throw shifter and boost gauge.
6. No towing....at least that is what the manual says.