1. I doubt very much that any competently run datacenter has much more than 100% required capacity. The people who design, pay for, and run them aren't fools.
2. In a datacenter, when (not if) the grid fails, the UPS is not to let you perform an orderly shutdown. It is to give you the time you need to fire up your on-site generators, including some "oh good grief why aren't they starting?" time. Or so the people who run the datacenter my employer uses tell me.