I had an occasion to fly at 9500' yesterday to compare speed pre-yaw damper vs post-yaw damper.
Pre-yaw damper, cruising just barely LOP at 9500' I would see 157 KTAS. If I remember right, fuel flow was about 9.0 to 9.3 GPH. That speed was calculated using our TAS indicator by aligning the OAT with altitude and reading the value.
Yesterday I did the same thing. Fuel flow yesterday was 9.1 GPH. Using the airspeed indicator I got 160 KTAS. Using the TAS calculator utility on the GTN650 I got 161 KTAS.
So it appears that the yaw damper is about a 3 - 4 knot speed mod.