Moving to the hardware forum.
There is absolutely no difference between two GeForce cards in SLI and the same two cards without SLI connection for Adobe's GPU Hardware acceleration. Their could be complications with using them in the SLI mode as Adobe does not test that mode.
Yes and no!
For renders to DVD, GPU horsepower is very important, so yes, two GPUs would be just about twice as fast as with a single GPU assuming you have a strong system (probably a good assumption if you are considering dual 980s). Both cards will be used by Premiere, and SLI is not required for them to be recognized. Bill is correct about Adobe not testing this mode, but at least one user on the forum reported that dual GPUs were recognized with and without SLI enabled.
For renders to h.264 (Blu-ray, etc.) however, speed will not be improved as you will be CPU "bound" for those exports, not GPU limited.
Do you have at least:
- 6-core or 8-core i7, overclocked to 4.2+ GHz or dual Xeon 10-core E5 v3 or better, and
- 64 or 128 GB RAM, and
- Sustained transfer rates on all volumes of more than 400 MB/s, and
- 32+ unused PCIe-3.0 lanes available.
If the answer is YES to all 4 points above, then a dual GTX 980 may - just may - be beneficial if:
- you render for previews all the time using accelerated effects, or
- you export from source to destination with rescaling (i.e. 4K to HD or HD to SD), frame rate changes, frame blending almost always.
If your workflow does not include these two points almost all the time, having two video cards is a waste of money.
Using SLI is irrelevant for PR, since the video cards are treated as separate components. If anything, using the SLI bridge can only complicate matters and reduce performance, because of the inherent overhead of the SLI design.