Hi everyone, 
I used to test some stuff
My curiosity push me to ask one question
I'm using an SLI of 1080 (one is EVGA FTW, the other a FE because I get it free so^^)
Both get +130MHz on GPU clocks and +500MHz on Memory clocks, and as linked they behave the same clocks so
I used to watercooled them both, using 2 standards sli soft ribbons bridges, because GPUs dont have same heights :/
I have absolutly no issues to run it, pure bliss on my PG348q monitor, FPS are so high its just amazing comparing to single GPU
 
The point is; 1 card (main) get 96-99% in full load, the other one 85-90 sometimes 95% 
Not a big gap I agree
I tought I could have a CPU limitation, some kind of  CPU Bottleneck but my 4.3GHz i7-6850k on my Rampage 5 Edition10 is about 43-50% of load durings test
This case in particular in BF1
 
In Unigine Heaven, Valley or others, both  GPU a full a 99%, CPU usage around 30% load
 
Is that GPU Usage "limited" by game engine, or anything else?
 
Thanks for your lights ;)
 

That is simply how SLI profiles work. You will never get full utilization while gaming on both GPU's.
 
Better SLI profiles provide upwards of 90% utilization from the bottom card but I havent seen much higher than that.

I disagree with that. The number of users with SLI is so small, even if developers put effort into, the ROI will not be there (even if the number of SLI adoption doubles). Most gamers will buy a card, use it for 3-5 years, then upgrade to a new card that runs quieter and more energy efficient, and probably most cost effective than buying an older card to SLI that may require a new power supply to power the card, not to mention an used card without warranty, dirty/dusty, may or may work, heavily OC previously, or possibly near its end of life. Coupled with nVidia lack of support for SLI, I think SLI will be gone in 3 years. And probably not needed too with better and newer single cards coming out.

The Unigine benchmarks are both pretty CPU limited with SLI.  Heaven isn't quite as bad as Valley, but it's definitely hindered.  Valley is horrible, though.  

chrism101
one is possibly doing the Physx portions and the other is matching the non -physx load of that GPU... just an uneducated guess?
 
More uneducated guessing- the GPU that is running slower is Hotter and is experiencing underclocking?

Could also be the game that is being run. Only some games leverage both cards and those that do can see a large improvement. If one is running hotter, then both would also slow down as SLI keeps them at the speed of the slower card.