Hi. Which is the factory unit you are looking at?
I am using a soundplicity 3 and sirrus sca1 adaptor. The sound is good from XM and from the BT for streaming audio. However the volume balance between I put inputs is off. The XM level is much louder than BT. The SoundPlicity does let you adjust level into BMW head unit but not for across the individual inputs (BT, XM, USB,Aux). The streaming BT music is good but call BT phone calls are not great. Overall I like the unit and support team at BAV are solid.
http://www.bavsound.com/Device-Integration/
Other small point is that the menus for XM channels stinks. Does display what station info and song you have on the screen. You can save 8 presets but changing channels is done one at a time. Solution works great if you have a few regular stations but not good for when guests want to scroll through.
Overall rather good for the price and convergence of tech but far from factory looking.
Leaves me thinking of fixing my BT and exploring factory SAT.