Andrew, if to looked up correctly, we are talking about Windows and BOINC 7.4.42.
Not sure if vbox version of BOINC is the way to go for GPU.
As Gary mentioned, make sure you have enough CPU cycles free, a single core should do.
I'm getting ~95% of HD7950 GPU usage, running short GFN, i5-4670.
3 cores are running GFN on CPU, 4th core is feeding GPU (4-8% CPU usage)
I would also try latest Catalyst driver (just released 15.7 is working fine in my case)
Badge score: 1*1 + 5*1 + 8*3 + 9*11 + 10*1 + 11*1 + 12*3 = 186