Is 16GB of RAM overkill for a Pi-hole + Unbound setup?

Yes, 16GB is overkill. 8GB is more than enough — Pi-hole and Unbound idle at 200-300MB of RAM and rarely hit 1GB even under heavy use. The bottleneck is the Atom x5-Z8350 CPU and network latency, not memory. Save the money for a UPS or faster SD card instead.