Ultimately I wanted to create threads at blinding speed, but my primary intent was to find out how far I can reach at all. With the current implementation, there is no large advantage for Stackless over the traditional calling scheme. Normal Python starts a recursion to a new interpreter. Stackless unwinds up to a dispatcher and starts an interpreter from there. This is nearly the same.
Real improvements are there for implementations of coroutines and threads.
They need to be simulated by classes, or to be real threads in Standard Python, while they can be implemented much more directly with Stackless. Much more improvement of the core doesn't seem possible without dramatic changes to the opcode set. But a reimplementation with more built-in support for continuations et al. can improve the speed of these quite a lot. Specific applications which might benefit greatly are possibly like Swarm simulations, or multi-user games with very many actors, performing tiny tasks. One example is the EVE game which is under development, using Stackless Python.