Get some test pipes. They will increase the volume and give you a noticeable power bump. Best mod for the M3. You will get a CEL so you will need a tune or a CEL fix (like Akra delete-R or BMS downpipe fix). There is no reason to buy a new x-pipe.
Removing the resonators won't make it much louder, just more "unpleasant". Resonators help to remove some unpleasant tones more than they muffle the sound (hence the need for that big ass muffler).
2008 IB E92 M3| BBS | KW | Arkym | Platte Forme A.G. | Active Autowerke | K&N | Fabspeed | Dinan | Evolve-R