For the past several months when using a Tacx Genius in ERG Workout mode the Wattage one ends up riding at has been about 7% too high.
However, on Friday I got a new version of the Tacx Genius firmware from Tacx; version 1.1.325. Not sure why this hasn’t been release yet (probably still testing)but this completely resolved the issue. The Watts in workout mode are now pretty much spot on.
If you are have issues just ping Tacx on FB and tell them ERG mode is not working and they should send you a bin file you can use to update the firmware.
As an added bonus I often found that I fighting the trainer to hold a higher cadence. With the new firmware holding your power output at a higher cadence seems much easier.