Just posting from experience - there are two morgans on the dressage circuit around here. One is fairly solid in third level and the other is beginning second. Neither do markedly well, but if they are solid in their work and very steady they will get 60s. The classes are then being won by scores in the 70s though.
Overall, some judges are going to view the horse's movement as a fault and mark the entire test down movement by movement, and other judges are going to accept it as a conformation thing and score the test according to how the work is performed, and will mark down only in the "gaits" portion of the collectives.
And unlike Spyder I don't believe that it is the extensions that cause a lower mark. Many people view the movment of these horses (fresians, morgans, etc..) as being "out behind" and not truly carrying. And with the high knee action, many horses then trail behind and do not match the knee action with hock action.
If the work is good and correct and not "breed-y" looking then the horse will do fairly well. If the horse comes in looking like a breed show horse with the excessive knee action and nothing going on behind then don't expect much.