When i had my RS. I had 3inch diff back cannon.

Had the rumble, loud as fuck though.
Got a custom system made from cat back.
2.5" from cat to hot dog muffler.
2.1/4 from hot dog to superturbo muffler (oval muffler) finished in 2.5" tip.

Improved performance. Sounded awesome. Only cost $300.
Dont bother with much else as i tried. A mildly worked 2.5 N/A will get trumped by a wrx in a straight line.

Get some driver training classes, handle what youve got now. Then throw another 150hp at it in a wrx.

__________________
2016 Subaru Outback - Work Wagon
350NM of CVT Goodness
Pit Crew, shit talker.