Thanks to @dlang and @JDogHerman who both took apart their motors (three times for David) so we could count the gear ratio and @blurfl for his expert assistance it looks like the gear ratio is actually 289.776234568:1 resulting in 8113.7345679 steps/revolution. (i was close empirically with 8114).
Changing the gear ratio in GC should to this value should solve the issue.
You can find the discussion and photos to perform you own teeth counting here: