I'd go back to stock x-pipe, see how that sounds. If that's still too loud then yes - go to a quieter axle back. Power gain is from primary cat removal rather than secondary cats.
I've got a Dinan axle back w/ test pipes...sounds awesome...quiet when I want it to be, loud when I want it to be
[08 e90 M3. Jet Black/Black. Dinan Rear Exhaust. ACM Test Pipes. ESS Tune. Eibach Pro-Kit + e36 bumpstops. 18x10 Apex Arc-8. AA Air Filter. RPI Scoops. CREE Angel Eyes. OEM Blacklines. Black Reflectors. Gloss Black Grills. Matte Black Gills. ZCP Shift Knob]