In the mustang world we have MIL eliminators for off-road x pipes that eliminate the cats. Essentially they just jump out the connection so the computer thinks everything is working as designed. Not sure if BMWs have something similar.

As for removing cats for performance gains - yes, there are gains to be had. Cats are extremely restrictive. A more environmentally friendly mod may be to find a set of "high-flow" cats that flow better.

IMO the gains don't justify the hassle though. Maybe on a street/strip car, but not on an X5.