I can see installing a valve if the sump is on the next floor under (ie basement) because the plumbing can hold a lot of water over a long distance. If you need to slow the flow through a sump, put the valve on the pump. The drain cannot work any faster than what it's fed - simple law of gravity. And even in the scenario I just mentioned, if the power shuts off and there's nobody home to shut the valve, if sump isn't sized properly and maintained at the right level, there's going to be an overflow no matter what.
If the overflow box is correctly installed and not leaking, with a proper standpipe, there's only a small amount of water in the plumbing if the power shuts off or is shut off, and that should drain in a few seconds. So I really don't see the need or the logic in putting a valve on the drain. And if you're hard plumbing - valve or no valve, better put a union in there so you can take the thing apart without having to cut pipes.
When the system is running, IMO it's dangerous to partially close a valve on a drain (hence I think it's not a smart thing to do)... all it takes is to choke that drain just a leeeetle too much, and your main display will overflow because the drain cannot keep up with what the return pump is pushing back topside.
I've seen displays overflow from hang-on siphon boxes that get a bit hung up because of an air bubble or whatnot, slowing the flow downward, *just enough* so that the tank over-fills, and then overflows. Same logic applies to a gravity fed overflow drain.
That's why the drain pipes are always wider than the return pipes - more allowance.
Jenn