Am I just in over my head? Is it unrealistic to want him to compete seriously against the collies and aussies that inundate the agility world?
It depends how you define "compete seriously."
All else being equal (health, fitness, handler's skill, etc.), you're probably not going to beat a BC in agility with a GSD. The sport is pretty much made for BCs.
I think your trainer's attitude was pretty rude, but I've encountered the same attitude among the most Seriously Serious in the sport. If you're coming to them with an "off breed," there are some instructors who will be dismissive of your team. It's the same in any sport, though. Show up with a Doberman at an IPO club and you might encounter the same attitude. Bring a bull terrier to a competition obedience class and prepare for scoffing. If you have an even slightly
"off breed" in your sport, some people are going to be dismissive about it, because some people are just jerks.
My advice is to ignore them. If you like the sport and your dog likes it and you're enjoying the work, then keep going. Find another instructor who has the sense to welcome a newbie into the sport. Keep training. Compete if you want to compete, but remember that the purpose of the sport is to develop a partnership between yourself and your dog. As long as you're both out there enjoying yourselves and working as a team, IMO, you have