It's not really your fault that you bought the 1866mhz memory since there's so much website hype out there to sell the modules, it would have behooved you to have investigated the 1866mhz claims at AMD itself.
For the latest performance benchmarks and detailed technical documentation of the AMD FX Processor, please visit Benchmarks and Technical Documentation.
*1 = 1866mhz supported with 1 DPC (DIMM per Channel) in single memory slot motherboard designs