[Oblarg]

Motor distribution and shifting aren't really the primary drawbacks of a mecanum drive; it's the decrease in traction that gets you. There's little purpose in shifting to a lower-gear with a mecanum drive train when your pushing ability is traction-limited.

As for your idea, I'm having a hard time visualizing what exactly it is you're suggesting, or how it'd result in a noted improvement over the standard one motor and one gearbox per wheel.