Can I use 8GB 1666 MHz DDR3 on Macbook Pro Early 2011 15?
Yes you can use.

They need to be SO-DIMM DDR3, if your CPU doesn't support 1666 MHz they will be clocked down to 1333 MHz.

About your model (Macbook Pro Early 2011 15 inch) looks like the 2.0 GHz CPU version supports only 1333 MHz, versions with 2.2 GHz or 2.3 GHz CPU support 1666 MHz