have you tried to compare 970gtx alone versus the 970 + 750ti? i am curious on how the pair up would work out for you.
I think the performance boost is minimal , given the increased power usage from the 2nd card
I could benchmark the 970 alone, but that requires fog and debris turned off since the 750ti is a dedicated physx card for me. So it wouldn't be a apples to apples comparison.
Edit:
Here are my results at 1080, everything max unless gameworks stuff noted below
970 only/All gameworks off
min 64 avg 92
970 only/enhanced rain and light shafts on
min 60 avg 91
970+750ti/all gameworks off
min 60 avg 92
970+750ti/enhanced rain and light shafts on
min 63 avg 89
970+750ti/enhanced rain, light shafts, interactive debris and paper on
min 63 avg 88
970+750ti/enhanced rain, light shafts, interactive fog on
min 54 avg 70
970+750ti/all gameworks on
min 48 avg 66
In conclusion, on my system, adding the 750ti for dedicated physx does nothing except allow me to turn on fog and debris options. Those options aren't available while solely running the 970. Other than that, fog is the only gameworks option that truly puts a dent in performance. So if you are running just a 970, there is no harm in enabling the enhanced rain and light shafts options in gameworks.