The ignition cap is built into the engine harness on the BMWs, and I have never seen one fail, the issue with the Jags isn't that the capacitor itself fails, but it will get a higher then normal internal resistance, and cause a voltage drop to the coils' power feed(the unit is wired in series with the power, but the cap itself is in parallel)
As to the fuel trims, assuming the K&N is sealing correctly in the air box(is this a drop in, or cone style? - same thoughts apply for either), it shouldn't make a difference with fueling, as the engine dictates how much air passes through it(assuming the paper filter is not clogged) That said, due to design, and the K&N being able to flow a large volume of air, I could see some turbulence being created, slightly throwing off the MAFS reading.