Just out of interest - a late comment.
Here in the Uk, there have been only 2 PGM-FI variations for the ABS. model. The pre -2008 version and the 2008 and later models.
We had new models in 2008, 2009 and 2014. As far as I can make out, all had the same newer PGM-FI umit.
I've had 2 ST1300s from brand new.
The A6 always had a high pitched rattle from the engine, like feint jangling keys, when hot, accelerating hard, at higher altitude or with E10 fuel. It was pinking. Pre-ignition. So the knock sensors weren't doing a good enough job, or the ecu was ignoring it at that level.
For certain it wasn't the same degree of pinking that you get with the old single cylinder two stroke fixed timing systems.
I eventually stopped it by using higher grade fuel and curiously 10w40 fully syth oil. I found the 10w30 to be too thin in the alps.
But this engine always struggled for low down grunt when negotiating tight, steep uphill hairpins. It required first gear and a lot of clutch slip to prevent it from stalling.
In contrast, my 2009 model has always felt superb. Gruntier, no clutch slip required - just put it into first and it powers round the corners no problem - and has never pinked.
My feeling is that the A6 was designed to run a tad too lean.
I don't know if this analysis is correct, but I know I would never look at a model that was older than 2008.