Yes training is surely the key to progress but there are also some physical and mental limits. Some horse can't physically jump well a whole course above a certain height for example, some also don't have the mind to do it.
So yes feeling is crucial, when you know your horse you know when it's too much for him. Your trainer and his/her experience will also be keys to understand your horse's limits!
I agree with Melinda, however when you have your horse, it's natural to want to know where you will be able to go with him. I think it's all about feeling, you can feel when your horse do things without any difficulty. Of course it's not because your horse struggles that you have to stop, with training and maybe some changes you will probably succeed!
It's hard to talk about a horse's limits, for example one horse can seem to don't evolve with one rider but maybe it's the couple that can't evolve, not the horse or the rider.