Overdriven pulley how much is too much?

Overdrive will be better almost all the time. Won't cause a single issue. Make sure the radiator is clean and suited to the application.

I've got a flow kooler wp, 5 blade fan, shroud, Hayden clutch, champion 26" alum radiator, and stock pullies and my 10:1 408 with headers will creep from 195 to 210ish if I stay idling. It drops back to 195 as soon as I move forward at any speed or if I rev the engine. I'd bet over driving my pullies would keep the temp solid.