Well, I finally got chance to adjust the belt today, 1/2 turn of the screw slacker and the belt looked quit limp!... so, with discretion being the better part of valour I wimped out and only slackened it 1/4 turn which must mean it was pretty good from the factory/bike shop I guess. I was surprised that even a tiny movement seems to make a huge difference to tension.
@leifg was right, at 2 turns looser the belt would probably have jumped right off the sprockets!
I've just been on an exploratory ride around my normal shopping loop and still didn't experience any negative effects, everything is really very smooth
Between recalibrating the hub, reducing the startup gearing in the app and slackening the belt tension slightly, it FEELS like the Automatiq is now responding much faster and accurately to changes in torque and cadence. The sensation that the hub is being indecisive about what ratio to select hasn't recurred... yet!
So far my testing has all been without assistance, so I don't know if there will be any positive effect on battery milage (when I come to use it) but I can't imagine it'll have made it any worse... I think I'm just gonna leave it now and just use the bike as it is.
Thanks to all for their help and advice