I wonder if imposing a minimum time spent on other activities (e.g., bike riding, jumping on a trampoline, swimming, board games, reading) would be a better and less "controlling" method than imposing a maximum time spent with gadgets? I suppose, though, that this probably wouldn't solve the problem with their aggression when switching "spaces", unless perhaps they found another activity that they loved as much or more than their gadgets.

I'm not a parent and so I'm really not in a position to speak on this, but it seems plausible to me that the aggression is a byproduct less of transitioning from one space to another and more of having complete control in the digital world vs. having zero control in the real world. They might be lashing out because the digital device gives them the experience, however short and shallow, of basically being you -- being in control of most details of what's going on in your universe. If that's true, one approach might be to find a way to make them feel like they have more power and impact in the real world, without of course actually giving them too much control beyond their ability to wield.