From what I've read the 08+ STI uses a canbus computer system that is markedly different from the 04-07 STI, the ECU requires readings from a number of sensors placed around the body of the car constantly to run properly. If the sensors are missing, or not reading properly you are probably going to have all kinds of CELs and gremlins on your hands. While wiring it might be possible, unless you're a professional with a lot of spare time and knowledge I can't see why you'd opt to tackle this.
Personally I also don't see enough benefit from the dual AVCS to justify all the extra risk and headache that the swap might constitute vs a single AVCS ej255 or ej257. And this statement: "ultimately, i want a very capable, somewhat sleeper car that can tackle anything. mud, rain, snow, DDing, autox etc. a low key subaru that is cheaper on insurance, but will perform just as well as a new sti." doesn't seem to indicate a need to try something more complex than a built EJ25 with an upgraded turbo. If you're sold on the DCCD system, look at DCCDpro, it's an aftermarket controller available in manual or automatic for the 04-07 STI 6 speeds.
Also I believe 96+ USDM cars are all OBDII.
That's my 2 cents.