I do know that many teams have removed the final stage of the FP motor to get a significant increase in speed. They usually use a custom gearbox using the internals from the stock fisher price gearbox. (Though I've seen some modify stock gearboxes to make them work with less gears, with some success)
You could check the output speed of the window motors compared to the FP gearbox output. The window motor should have plenty of power for lifting balls with a straight drive from the output shaft as long as your belting isn't too tight. I'm just not sure you'll get all that much more speed out of it for the effort you'd go through to mount it instead of the FP.
Now if you happened to have an extra CIMple box or something else to do further reduction I'd recommend the CIM-u-lators.
Also, what is a Modulox Beatbox??
