The 2 pins more in the in the 8 pin plug are 1x ground and Sense1. Sense1 is only to inform the card whether a 6 or 8 pin plug is plugged in.
I think the big difference in your usage is that 8-pin plugs are able to pull 150 W, where 6-pin plugs only provide 75 W at max.
As far as I see the Sapphire HD 7950 needs 80 W at idle.
So I see 3 Possibilities:
Buy a ...
I'm not certain of specific APIs, but looking at the source code of an open source application that works with display resolutions or GPUs may be beneficial. Two examples I can think of are RDM and gSwitch (RDM hasn't been updated for a while but there are up-to-date forks available).