There are NOT very many Labs doing agility. At every trial, in the 22" and 26" classes I see a couple of Labs, a scattering of Aussie Shepherds and what USDAA calls "All Americans" i.e. mutts, and lots and lots and LOTS of Border Collies.
Most of the Labs are the (fairly rare) small and light version of the field trial strains. There are a few of the heavy conformation types competing, as well as some of the big rangy field trial types, but they don't tend to do very well - the conformation types are too slow, and the field trial types are too big for the equipment.
Shelley is the product of a conformation sire bred to a field trial dam. Her field trial grandsire is a very large dog, but she got his leanness and drive on a conformation sized body. It's an interesting combination - she has the low height, blocky head, broad chest, and more substantial bone of the conformation Lab, but she is lean and FAST, with the drive and "go" of the field trial line. She also, thank heavens, has a sense of humor and is just a "happy dog" - loves everyone and firmly believes that everyone loves her and is just dying for a doggy kiss on the nose or the ear.