people run e20-30 with frequency in their n54/5s, for the most part there havent been any real issues other than maxing trims...if youre tuned for it, you can run more aggressive mixes than those stated above. You dont want to throw a full tank of e85 in, but a modest mixture with pump can add power while smoothing out timing. Its really not that weird or exotic for the test driver to see how the car likes it.
E88 N54 Alpinweiss/Coral Red/Motiv HTA 3586r Tial .82AR/Other stuff...652WHP
F30 N55 XDrive EBII