Don't try that. It's very unsafe. An 8" diameter disc spinning in a router will be spinning many times faster than it was ever designed for. Most likely you'll see terrible vibration for a few seconds before the 1/4" shaft bends or breaks, which will then be much worse than the vibration.
Even with a small drum, the high rpm's will more than likely burn the wood almost immediately.
Just a guess, but probably the maximum rpm you'd want is maybe 1000, not the 20,000+ most routers have.