I know I'm not eml but find this quite an interesting topic so thought I'd put my tuppence in (as the owner of a spooky dressage coblet).
In my experience the more sensitive you teach a horse to become to a rider's aids, the less spooky they become. Once you 'get inside their heads' so they react to the slightest movement of your seatbones, they are so tuned in that a gun could go off and as long as you didn't react, neither would they.
I think of it as the true meaning of the 'submission' that the dressage judges look for. It's more than just obedience, it means that they are in a permanent state of readiness for your next cue, are completely attentive and will 'let go' of any emotional baggage they carry to join their mind with their riders.
The only problems you might get is when you put a less skilled rider on a horse with that level of training. Then it is not unusual to see them become confused, tense and worried.