Would a magna flow muffler give a rumble on a 07 subaru impreza 2.5i?

It is impossible to make a 4 cylinder rumble and sound like a V-8. A Magnaflow muffler will only make your Subaru a little louder.

Bob's right. Why don't you experiment by drilling holes sequentially in your old muffler until it sounds just right? Cheaper than trying various aftermarket ones. Only Porsche and Ferrari have gotten horizontally-opposed motors to sound interesting...and with a LOT of money!

Just make it a straight pipe, i have been doing it for years (i'm 67). Works great in all motors.

Except that reduced back-pressure may also result in lower rpm loss of torque, so be careful what you wish for.