A rear sway bar shouldn't take up any usable space, unless you store stuff under the car. I think you mean a strut tower bar. And that would take up a lot of space in a P5. The strut towers are buried under the rear seat belts about 8 inches into the plastic trim in the hatch, you'd have to cut a hole in the trim to mount one. But a sway bar that would be easy, however I've read that the MP3 rear swaybar can't be used unless you get the MP3 struts because they have reinforced end link mounts for the larger bar. The factory struts endlink mounts would probably bend or break off. However woith some inginuity a set of reinforced mount can be made that mount to the bolts that support the knuckle.SR Motorsports also sells a swaybar kit that can be used with the factory struts, I believe it's 19mm's.