Straight from the README for the latest NVIDIA drivers. Going to give this a try next week when I have more personal time.
Increasing 4-way SLI/Multi-GPU Performance
With some games and applications, you may experience little to no performance gain or
even a performance drop with 4-way SLI or multi-GPU configurations.
- Open the NVIDIA Control Panel, then click Manage 3D Settings from the navigation
- Click the Global Settings tab, then scroll to the Power management mode feature, click the corresponding list arrow and select Prefer maximum performance, then aclick Apply.