with those new mufflers the PSM still works?

 

The car didn't have PSE if that's what you mean?

 

In the 3rd to bottom photo in the OP's Post #1, the profile of the diffuser seems to show a nice size gap making the diffuser appear to "hang" from underneath the car. Truthfully this diffuser appears more cosmetic than anything else. With the motor and exhaust dominating the rear end, there simply isn't enough hollow space to take advantage of. I've driven MINIs in Germany with the factory works diffuser (BMW Motorsport part) and the difference in high speed stability is amazing (similar to attached pic 1). Those diffusers protrude from the rear end of the car as they create 2 deep channels of air. That's about the only way it would work on a 997 as well. Would be very DTM-like. Ultimately a real (functional) diffuser would require some cutting of the rear valance, and extension perhaps 50mm beyond the valance. The point is to channel air from as far back as possible, and exhaust the air from as high on the rear body work as possible.

OK, so someone make this for us

We're way behind the MINI world of tuning. M7 makes a perfect street part (second attached pic).

3rd attached pic just because it's cool.