Stock works perfectly fine if you boost it. Unless the motor is built and your fuel system is up to the task, you aren't going to reach the limits of a stock IM anyway.
Never hurts, but I wouldn't recommend just throwing money at parts hoping to fix the problem unless you know you have blown gaskets/exhaust leaks.Like Tanro recommended, look up how to complete a boost leak test on the car and that can at least rule out some potential causes.
If you want a cheap solution, a GST stock catback was a decent upgrade for NA cars. Otherwise, eBay is probably going to be the next best bet. Definitely stay 2.5" unless you are turbo, will sound like ass and will probably lose some power with the bigger exhaust.
I've had an Aeromotive in the past on my old 1g and it worked good. Have a Fuel Lab setup from STM on my 2g and no issues whatsoever. I recommend Fuel Lab just for the fact STM has a kit with everything you need for install.
It's flashing faster than it should, maybe a wire shorted somewhere? Will need to prod around the wiring and do some testing to find where the problem lies.