New year update
Whatever that EML was, it hasn't happened again. The car is running beautifully, way too much torque for the winter tyres though
Consumption is absolutely unchanged since I fitted the box. Trip computer is showing 5-10% more mpg, but calculating it brim to brim, I'm getting exactly the same 35mpg I always did.
The only question now is what next? Brakes don't need replacing for another (idrive estimated) 20k, so I'm loathe to waste all that perfectly good pad material unnecessarily. Suspension feels OK, but the car's on 93k now and it's probably getting tired. I quite fancy the M3 subframe bushes, but if I'm dropping the subframe I may as well fit M3 (or equivalent) ARBs while I'm there. That's about £300 on parts and a hefty labour charge - I don't fancy pressing bushes or dropping the subframe on my driveway.
Then there will be no point having all that fitted without replacing the dampers and springs, so there's another £600, but those I'll fit myself.