+1 to what mosc said. I'd base it more on what your future turbo ugrade may be. For me with the 2.0L I'll be heading the direction of an STi top mount with an evo3 16G. If I had a 2.5L I'd look into a TD06 20G with maybe a front mount. Either way I'd base it on what type of turbo you plan on, and of course if you stay stock a TMIC will be your best option.
hey longshot, just a friendly word here... the STi top mount is bigger than your stocker but not by much. I think it's about 40% larger? The 06-07 WRX's actually have a smaller intercooler than the 02-05s. Anyway, A good aftermarket unit is going to be 2.5x your stocker. An E3-16G is a nice medium turbo that works well with a TMIC but it will limit on an STi TMIC. I'd be looking at Perrin or APS or even SSAC if you want something cheap.
Not necessarily. Especially if you go with a TMIC. It depends on what numbers you are planning on or trying to make on the car. You will probably be limited to ~400AWHP with a top mount. Those are just numbers I have seen and read with a top mount.