Hey Meisters,
Like yourselves, I am a fan of the CX12. I've bought some in recent months and retuned them, and they are really airtight - I think more so I remembered them being before.
One thing I've found with the black cover is that the lips can drag on them, producing friction. Normally this wouldn't be an issue, but recently I've been doing a lot of tongue blocking things on it, and one day the corners of my lips were pretty raw afterwards. In fairness, this could also have been from playing tongue-blocking some older 270s and the Hohner XB40 around the same time.
I have the CX Jazz cover but not the inside of that harp and I find that the lips move more easily on it, due to whatever glossy coating it has. It isn't *quite* as airtight as the black cover (almost, though - not bad in general) when combined with a standard CX12 body. This probably isn't to do with how far in the mouth it is or the black cover being the deciding factor in the embouchure, as I've experimented with different embouchures, pushed further into the mouth etc. The Jazz cover may well fit the Jazz body a little better. My feeling is that Hohner have changed tolerances in general, as the recent CX12 slides didn't need to be taped - in fact the slide wouldn't move very well when it was taped. All in all, I'd say it's a good solution for people who don't like or have problems the shape of the black CX12 cover.
Eugene