Until someone tests it, no-one knows what speed 1866MHz ram will run at in a 2012 iMac.
It *might* run at 1600. But it *might* run at 1866.
It all depends on how Apple have implemented the chipset and EFI. In some previous Mac Minis 1866 ram ran at 1866. I believe in the latest Mini it runs at 1600.
Someone needs to test it.
There is no chance of 1866 being "standard" on the next iMac though, since anything above 1600 is higher than the Intel spec and Apple will not release a computer that runs out of spec as standard. No chance.