Yep: Ke=1/2mv2 Were it me I would probably just stick with M/s and grams, but unit's don't really matter.
Functionally you have a couple of issues though. First, there's a spin up period to the router, and you don't know where in that RPM ramp up the disc let go, so you don't actually know the RPM. (for that matter I've found the RPM ratings on the stickers to be more hope than fact). Then pinning that specific piece back to a position in the 9" disc would be very difficult without some very good high speed footage. And if you had that you could just measure the velocity on screen ala Mythbusters.
You should be able to pretty easily get a range though. You can calculate the energy at which the disc won't fail (what it's rated for) and the max possible energy that router could have imbued, and say "it was between x and y Joules".
It's also worth noting that guards and being careful around power tools are a great idea, but the key take away is don't bolt a 6,600 Max RPM disk to a 30,000 Max RPM Router and turn it on.