Well one re-install and one restore of Windows later I can report that GPU Tweak's Monitoring overlay (both Widget and Standard) is causing severe frame rate drops each time it updates.
After noticing gameplay smoothness feeling horrible, I tested using Heaven benchmark, BF3 and Project CARS - all of which were showing similar signs.
TLR - if you have GPU Tweak's Monitor anything other than closed, you will be hit with a frame rate drop each time it updates.
For those of you that love graphs - here is a frametime graph for a 10second period. (from top to bottom) Monitoring off, Monitoring on (1sec interval), Monitoring on (3sec interval)