Hi.
Can these Haswell NUC's take 1866mhz RAM or will it just downclock to 1600mhz?
Max officially supported speed is 1600MHz, so I'd imagine they'll downclock to that. There is little real benefit to 1866MHz on this kind of chip anyway (memory bandwidth isn't the IGPs bottleneck).
The Broadwell HD 6000 and Iris 6100 chips have official DDR3-1866 support though, and there are NUCs available with these now.