What you describe is called a soil interface, Grey. Roots don't stop, but they do stall when they come to one and that bathtub effect can cause trees to tip in very windy conditions, especially when they are young. Eventually roots will cross the interface, but it can stall them for a year or two. Mixing the soils with the compost can help some, but now we have another soil interface. So there is the root ball to compost interface, then the compost to clay interface. Simply planting in the same soil and allowing the nutrients to leach into the soil creates one less interface (only rootball to clay in the other model)...
For veggies and such, with smaller roots, creating the nice soil is more beneficial. But for trees and shrubs, no ammending and improving soil from the surface down is the best approach. Opa is right again...
HG