Well I own a 690 so I can confirm that the reference cards "Are" indeed voltage locked.
As for a decent OC for one. Well of course no card is alike so you may have been lucky or unlucky with yours, it may OC really well or it may not.
As a point of reference I have mine stable at:
GPU Clock @ 1095Mhz
Memory @ 1597Mhz
Boost @ 1200Mhz