Can i put a 02-06 wrx or sti cat back exhaust on my 06 impreza 2.5i????

the axle back will fit no problem but the catback wont fit unless you get unequal length headers and a new cat pipe (like the stromung). that in my opinion the best way to get proper subaru boxer rumble out of the car (what you hear from the turbo cars, unless they're running equal length headers, which're in the minority). the wrx/sti mid pipes (from catpipe/cat/catalytic converter to axle back (just the muffler with attached pipe)) are different from ours, but once you get the UEL headers (either the borlas or the ebay borla reps) and the cat pipe you'll be fine. check out nasioc.com or rs25.com for more info. both are free to join

how will it sound with just the axel back??

Or, the cheap way, you could take it to the shop and have them "replicate" the cat on WRX's and rotate the flange for you. But like the previous guy said, you will not have the "boxer" rumble if you retain your headers.