While I don't know much about overclocking gpus with a custom bios, I'm fairly confident you don't want to pump that much voltage into a gpu. The stock 980 only uses I think 165w. I assume it's the same with a custom bios as normal, so use msi afterburner and up the clock speed until it's unstable and then add voltage incrementally until it stabilizes. You most likely won't add enough voltage to damage anything, unless it's crazy like 400. Longevity won't be affected really, frankly it'll be old enough you'll want to upgrade to something new before overclocking it kills it.