Should I add another 8GB stick of RAM or replace both with faster RAM?

Yes, just buy a matching 8GB stick at 2666MHz. Don’t replace both with faster RAM. Adding a second stick gives you dual-channel memory, which boosts performance 10-20% in CPU-bound tasks — way more than the 1-3% you’d get from faster RAM. Faster RAM only helps if your CPU and motherboard benefit, and many systems top out at 2666 or 2933 anyway. Get the exact same model to avoid compatibility issues.