I have the same model GLA as you, also registered in Oxford - the place where I spent some wonderful years when younger. I look forward to hearing what you think of your new pocket-rocket. When it's run-in, put it in sport mode, knock it down a gear to spin up the turbo, and then see what happens when you plant your foot to the floor
In theory the manual mode will give you the best control when your head is in 'sport' mode, but the car is so quick that as soon as you've changed up or down, it's time to do the same again. It's best just to put the gearbox into 'sport' mode and let the car do the changing. And if I find it difficult to keep up with the liveliness of a GLA250, what must it be like for the AMG45 owners whose cars make our GLA250s look positively pedestrian? Is there any point in them having a manual mode!