Hi, I have had my A3 40 TFSI e for about 6 months now and have just done my first long distance run (200 miles each way) with no easy access to recharge while away, so I ran the car in Battery Save Mode most of the time - still uses the battery for initial pulling away but then switches to the petrol engine and stays in petrol mode for most of the time - I only switched to electric mode when in the town centre. So all of the motorway running was just using the petrol engine. I found the drive very good with no concerns about the performance - only niggle is the tyre roar especially on some of our crappy concrete motorway surfaces!
A few other comments on using the different modes...
Day to day, I leave it in pure electric mode for all local journeys (up 30 miles total). For slightly longer days out etc (up to say 100 miles total) I switch to hybrid and then for the longer runs use the battery save mode, perhaps switching to hybrid on the way home to use up any remaining battery. All works very well and the switching between electric and petrol is totally seamless and you often don't know which is being used, unless you look at the dashboard!
I had been running most of the time in the Dynamic setting (rarely have I also used the Sport mode on the gearbox as that does make the petrol engine a bit thrashy) but for the longer trip I have been using the Comfort setting and I'm tempted to leave it in that now.
I also purchased the Adaptive Cruise Control via FOD. This worked very well on the long journey, but I would suggest using it in the Comfort setting - in the Dynamic setting it will get you back to the set speed as quickly as possible when you signal and pull out to overtake, which on a busy motorway I found too aggressive at times.
BTW, in the battery save mode over the longer distance, it quoted about 55 mpg.
Anyhow, those are my thoughts and overall I am very happy with the car, having come from a nearly new BMW 420i.