I've noticed, that some GPU tasks makes computer very sluggish (for instance PrimeGrid WU's), and some not (Collatz Conjecture WU's).
If in the Boinc Manager was an option (check box) "Allow exceptions" (under option "Use GPU while computer is in use"), active only when check box "Use GPU while computer is in use" was unchecked, and a list with exceptions, when we could select GPU projects (exceptions) to run even when computer is in use, then we can put projects with small impact on computer performance onto this list.
During normal using of the computer, only tasks from projects on that list would be active. Tasks from projects not included in the list would be active only in the computer idle time.
That solution could be a temporary remedy to the lack of precise GPU usage control.