It sounds interesting. If the engine behaves different/better I would suppose it is due to the reason that the coil gives a better spark. Do you have a clue what amplification the different coils have?

A little bit off-topic, but I have started to test and sell a battery performance unit for trucks in Scandinavia. The impact on the battery lifetime, capacity and cold start properties seems to be undoubtfull. However the unit does also induce a high frequency ~1,5 V amplitude which is supposed to have a positive impact on the ignition process on petrol engines. I haven't had time to evaluate the ignition performance in detail, that is why I am interested. I have also some other ignition ideas in the pipeline and I would say it is a shame to just take some VW-parts off the shelf to tune your Q4, when the world is full of more advanced concepts that are not too complicated to implement. Send a PM if you are interested in the discussion of alternative solutions.

By the way, the battery unit have so far reduced the average fuel consumtion in my 147 1.6 TS with 4-5 % in an A-B-A-B test over 7000 kilometers!