People should read what you wrote b/c it seems like there are a number of 2.5i owners buying and installing the WRX SPT exhaust.well if youre scared that 3" will rob you of power then im guessing you have a 2.5i. For the rumble, you need UEL headers. for the deep sound, well what im going to get, and what everyone recommends, is the stromung axel-back. their cat-backs wont fit our cars, so you have to custom mod the piping from the cat to the muffler. also ya 3" will pobably rob you of power/ low end torque. people here seem to think it wont but a lot of the respected experts over at NASIOC say even 2.5' is on the big side and that 2.25" is optimal (Stromung makes hteir exhausts 2.25" for N/A subies and they know what theyre doing).
ya im sure you'll love it over the the stock and way restrictive pipes, i heard an HKS single system in portland this weekend and it wasnt too loud when he got on it and it had a good rumble at idle. not sure if was resonated and muffled or just the muffler but it was cool!!!ive already decided to get a 3 inch single pipe catback with an HKS hi power muffler gettin it installed next week!!! pretty stoked to get it put on