Dave
There wasn't a thousand foot of QNH/QNE difference, QNH was around 1014 as I remember. I've flown a few different GA aircraft with similar transponder inaccuracies. You know what's coming when ATC say 'G*** repeat altitude?'...
I weas under the impression that the big advantage of an encoding alt was it sent the mode charlie corrected for whatever you had set on the subscale. I'm not a tech head so I'm probably wrong and stand to be corrected.