I am running BOINC 7.16.5 x64 on Windows7, with PrimeGrid project attached. When not snoozing GPU, my computer's too laggy to use, but snoozing it wastes a lot of computing power. So is there any way to limit the GPU usage to, say, 90%, without snoozing it?

Sadly, none that I know of - and nothing has been suggested by any of the GPU programmers that I've spoken to. The design of the programming environment for GPUs seems to allow any GPU to grab, and use, any GPU resources it can: there doesn't seem to be any control interface exposed to the programmer to limit that.

Mind you, the programmers I was speaking with were intent on getting as much performance as possible out of the devices: they may not even have looked for a way of slowing them back down again.

If you get a secondary GPU, you could.
There are a lot of older PCIE graphics cards for sale below $35 (like a GT730), which run a GUI just fine, as well as play back 1080p video footage.
One step up in the $100 range, gets you a GT 1030, or RX 550, both do basic 3D gaming quite well.
You can disable one GPU, and crunch on the other.

Though once you'll go down that slope of adding a GPU, you'll want to add it for crunching as well, believe me ;)

Really a good idea. Got it.