It appears that all three recipes (normal, fast, express) create the same entity (belt-balancer). The items created are no longer distinguishable - they all show up as "balancer part" in the same stack. Does this mean that all balancers are automatically upgraded when the next tier is researched?

The Balancers are running always with the speed, that is needed for the input and output, that are used. So no new Entity is needed for higher speed.
The recipies are there, to have a better output for balancer parts, when you unlock better belts :)