Well you know my opinion
....
I have been on the fence to replace the OEM headunit and ended up not doing it. Besides loosing the BC you'd also loose the capability to set time/date etc.... after a power loss (all done via the HU) .
I personally would probably first go with the OEM HU (I am assuming
you have the NAV unit) and maybe later down the road you can always add a new HU (and you can always move the OEM HU into the glove box or somewhere else)
As has been mentioned by Kevin/el_duderino you don't need to use a CleanSweep if you have DSP as you can remove the DSP amp and revert the system to a non-DSP setup.
As for installing an amp or sub in the cargo department. As you've probably seen with my setup (
http://www.janixworld.com/x5/Stereo/) installing a 10"/12" sub into the factory location can be a very challenging build. Especially if you want to go the 12" route. As for amps the easiest route would be to just hide them under the floor cover and rebuild the insert with something more durable. In my case we've rebuild the entrie loading floor which is very time consuming, it will look great but requires a lot of work and knowledge to make it look good and still be functional.
If you want to talk the folks who build my subwoofer enclosure and amp rack let me know and I'll get you in touch with them. They'll be more than happy to help and let you know how labor intensive it is to build a propper sub wooder enclosure for an X5 ....
- Jan