I don't see anyone bringing this up, so here goes...
Maybe it is not the opening rates but maybe the number of people flagging an email as spam in gmail that would cause problems?
I think that measuring metric would be a lot easier for google to clearly determine the intent of the user.
but even if it were being used, that would be a terrible metric. Just because a company has a terrible newsletter shouldn't punish them in the SERPs, imho