I noticed you mentioned "smaller" aussie. If you do decide to go this route I just wanted to give you my experience with this.
I had two aussies, that I loved dearly. They were really great dogs, very very drivey dogs, and they were ASCA dogs. Their father won multiple agility titles, as well as being the MVA (most versatile aussie) for 4 years consecutively. They were working dogs as are all ASCA dogs.
The OTHER route is AKC Aussies, they do not focus soley on work, and the ones that I have found have not been truly competitive in agility, or disc. They have been much more laid back than I wanted.
Now "Mini" first of all, this may be slightly biased because I am against the breeding down of a dog and ending up with a dog that is pocket sized but unable to do the job they were originally bred to do. My other reason is just yesterday at work we had a 7 year old 8lb Sheltie that was a "mini" that was in liver failure...he never grew because of this method of breeding.
Now that being said, I have met some mini aussies that are really drivey, and those dogs have been very intelligent. These dogs have also simply been bred "down" by crossing the smaller aussies together rather than the 40-50lb. Both of my boys were 40's. Then there are other mini aussies that have been cross bred with other breeds to create the minature version those dogs can be iffy drive wise.
If you're looking to become really competitive I think you could do it with a sheltie, I'll be getting into agility (hopefully) with Whiskey, but if you're looking for a bigger dog to compete with those black and white crazies than an Aussie is always an option. I included a picture of my two boys to show you the EYE.
Those are two very gorgeous dogs! Those eyes really get you~
