I don't really believe there are shelties who are well suited to tdaa. A slow dog is only going to be slower and more demotivated by courses that are tight and require them to be even more handler focused, slow, and cautious. It is counterproductive to run that type of dog in a venue where they must be cautious and overly attentive at all times. You can not move forward, grow, and expect more from the dog while you continue to ask the dog to be slow, cautious, and handler focused. I have yet to meet a Sheltie who would benefit from that type of set up. A slow dog will only be made slower.
I don't think any of us want a slow dog regardless of what our goals are. We may not all want the next up and coming world team dog, but I think we would all like a dog with at least moderate speed who can easily make course time.
While it may be easy to run this type of dog in tdaa because them often trotting or cantering slowly and being by ones side lends itself to completing these small tight courses, it isn't helping the dog to become a better agility dog. Courses that are more wide open and that encourage speed... Some usdaa and nadac, while not as easy for these dogs to make time, would actually be most beneficial for the dogs as their course types would allow the slower more cautious dog to pick up and really run without being cranked on as much. You are more likely to get speed out of a slower dog on a big flowing course with lots of room to run than on a tight course.
I think most of us strive to get the most out of our dogs, and a sheltie, be it a fast sheltie, or a slow sheltie, is never going to be able to give their handler the most and meet the full potential that is there running a venue that works against that. They are just big enough that it would be tight for them, making big moving dogs have to crank constantly and bounce jump, and slow dogs slow down more and watch their handlers more. It is because of this that I feel it is not a suitable venue for Shelties.
Besides that, I got a great mental image of my friends 20" bc trying to run tdaa. It would be hilarious. He ran his last jww at close to 6.5yps and is one of the most talented dogs I've met. I could see him crashing jumps left and right, body frantically scrambling to try to make turns and him flying past weave pole entries. A dog like him would pretty much destroy a tdaa course. Bar setters would hate him. No, I don't think the 20" dogs should be brought in at all.