I've been using this plugin for a while. Works great, no problems. However, I've set the max limit to 60 and this shows correctly on my Rcon, but when I play (with a high-end GPU) I get a constant 100 FPS (as reported by Steam overlay) so I'm just curious why it's 100 and not 60?

This is for the server FPS, not your client. What it sets is the target FPS for the server to run at.

Ahh right gotcha. So although my GPU is seeing at a rate of 100 FPS, it's only actually getting 60FPS from the server.