It can be done (I've done it a few times) but it's a colossal bitch! I'm 6' and 180 and I barely had room to slide under there. You have to work with your hands over your head (tiring) because there isn't enough room to work above your body. The x-pipe is heavy so it's difficult to maneuver it in and out while laying on your back with so little space. If you're lowered, it's even worse. At the very least, I would put the rear of the car on stands.
2008 IB E92 M3| BBS | KW | Arkym | Platte Forme A.G. | Active Autowerke | K&N | Fabspeed | Dinan | Evolve-R