Well here's a simulation showing what temps would most likely be like without anthropogenic forcing (the yellow on the top graph).
I don't know of any papers off hand that directly state the amount of the current 380 ppm of CO2 that's attributable to humans, but based on the past few hundred thousand years I would assume that without our influence it would be no more than 220 ppm.
So if we were neutral from the start reefs wouldn't be under threat from the climate due to temp or acidification. I'm not sure what kinds of limits other ecosystems have, but I assume they'd be alright too since there's not much departure from the recent past they evolved in.