I've written native C modules interacting with EVE online, probably one of the biggest Python systems in deployment, and they run on stackless python with a bazillion of threads for every little thing. While still needing to hit that 50fps window.
This is a common misconception about Stackless Python.
Stackless still has the GIL - it facilitates concurrent, not parallel programming. Stackless Python programs run on a single core, with cooperative task switching between microthreads.
This is "concurrent" in the increasingly common Erlang sense of the word:
"The real world is ’concurrent’. It is made up of a bunch of things (or actors) that interact with each other ina loosely coupled way with limited knowledge of each other." -- Grant Olson "Why Stackless" 2006
"Concurrent computing is a form of computing in which programs are designed as collections of interacting computational processes that may be executed in parallel." -- http://en.wikipedia.org/wiki/Concurrent_computing
The GIL was not a problem.