1. Self-levelling BMW wheel hubs?
They just self-level on all 4 wheels as the car moves/stops.
2. Carbon fibre mirror caps (AutoID UK, not the expensive BMW ones)?
3. Powder-coating the exhaust tips?
These are notoriously prone to deteriorate and the replacement carbon fibre ones are ridiculously expensive. A number of owners have got these sandblasted and then powder-coated. You should should be able to find an outfit that does this locally. Mine cost £50 the pair but feedback was I paid a bit over the odds, although I've been happy with the results.
4. Black Grilles and vanes
This is actually a photoshop'd image just to see what a black kidney grille would look like. I also virtually repainted the front vanes black (instead of grey). In the end, I decided not to bother. This was taken before I fitted the self-levelling hubs.
It also gave me the idea of just seeing what a body-colour grill and vanes would look like. As far as I know no one has done this. In theory you could take the existing grill off and separate the faux chrome, and spray it the body colour, along with the vanes. But although I did like the look - and it would be pretty unique - I decided to leave it as original.
Painting the vanes body colour is on my 'possible projects' list but at £25 a 150ml can from BMW, it's on the pricey side. Also, there is a cross-strut behind each kidney grille and some owners have taken the opportunity when replacing the grille to respray these struts in the body colour, which looks quite cool. Another 'possible project'