I have the ali top box , and another yes, mine takes a shoei neotec. You can put stuff like puncture kit, gloves, gps, inside the helmet and then inside the top box.
Fits a full face helmet but you'll have to take off the GoPro and the GoPro mount if you use one. In short it will fit a full face but literally nothing else.
I've just bought the Givi Outback Trekker 42 to replace the BMW top box as it's 42L instead of BMW's 32L but the Givi is more rectangular box with very limited height and was a massive waste of money. Going to stick with all BMW aluminium luggage and get them wrapped black.
Saw on here the other day that you can get the Givi coded to the BMW key if you do end up going to Givi route.
BMW really do need to review (and fix) the top box situation. The panniers are (as far as I'm aware) the same size as the vario panniers at their extended size which is great but the top box is dreadful and looks pathetic on the GSA.
Luckily the flap above be lock which you use to open and close the top box is faulty and a bastard to close so I can return it and get my money back.
However I'd already stuck on the back rest so that's lost now and the top plate is a total loss so I'm £160 out but better to be £160 out than £370 out.