I've found no information on the french and deutch Festool website.
I've asked to my Festool dealer, and he is not aware of the HEPA upgrade, but he will ask the question to Festool.
The Mini/Midi sold late summer 2011 and later should have a HEPA filter installed with the fleece bags. Also it bears repeating that it has the same motor as the larger CT models.
I realize that this thread dates back to 2012, but I noticed when researching which CT model I wanted to purchase, that both the Mini and Midi are rated for 130 CFM, while the CT26, 36 and 48 are all rated 137 CFM. I'm not sure how much difference that actually makes in performance, but it did help me decide to purchase the CT26, rather than the smaller units.