I would place a call to BMW of North America
Customer Relations:
1-800-831-1117
Let them know the bull that your dealer is handing you. What you wanted is what you paid for, nothing less. If you picked out a MINI that had the JCW Aero Kit or had one on order to be installed, that's what should be on the car. Someone screwed up and it wasn't you! You could check your order and go to the dealer, pick up an accessory catalog and stick both in their face. I know all the literature I have, all the pictures I've seen on line and the JCW's at my dealer have the vented shirts.
Before I ordered mine I emailed my MA 3 views of the Car as I want it from the U.K. website. It will be exactly what I ordered or I'm not driving off their lot with it.
If it's not what you envisioned you will never be fully happy with it. Don't settle for it just because they don't feel like eating the cost of the wrong parts.
As far as the: Mini reserves the right to make changes without notice. That should only apply when there is a design change for all further vehicles in the same line.
Good luck with this and be sure YOU are happy with the decision you make.

 

Might be too late, but here's some pics of my car after almost 3 years of use, with the factory aero kit..




More pics: /show/