Is there a way to control the clock speed of the GT 650M GPU in a Retina Macbook Pro from OS X?
If you don't need to use the MBP screen while using the external monitor you can disable it using a small magnet.

A small magnet placed on the left side of the MBP keyboard, more or less above where the SD card reader is placed, will make it think that the lid is closed. The GPU will need to do less work to drive just one screen (the external) and use less power, and with the lid still open the airflow will be beter.

I use this on a 2011 MBP, other versions may need to have the magnet placed at different places for it to detect a closed lid. And if you try this, please use the smallest magnet available.