I don't think you can train any dog to "like" whatever sport intended.
If they aren't into it, they just aren't going to do well no matter how much training.
My rescue gsd, we sort of fell into agility, and she loved it, you could tell she loved it. Obedience was boring for her, she did it, but she wasn't 'into' it.
Masi, I wanted to do agility with, but it was rather obvious early on, she just wasn't into it, sure she'll do the obstacles, but it's just something I can tell, she would rather be doing something else.
Put her tracking harness on, and she is raring to go, no stopping her..THAT is what she loves and does well at (even tho we haven't tested for it)..and obedience, she likes obedience
Aussie same thing, agility "ahh forgettaboutit", throw some sheep on the field and she is OFF the charts..
As for a gamble, I agree anything in life can be a gamble, I DO agree an older , even 6month + green dog can be a better bet than an 8 week old when having something specific in mind.
I'm sure alot of things come into play when it comes to a dog excelling at something, but I do believe, a dog will excell at "whatever" when the dog loves to do it. If the dog loves it, it is soooo much easier to work with, a pleasure to watch and a pleasure to 'play' with.
I also agree with Kristi, finding a gsd that is capable of high level agility competition is far and few in between, as well as having the longevity for it.