A very good solution!
We've found that the Tetrix axles to just be too wimpy to use for a drivetrain. Though we never tried a motor-diameter shaft.
This past season, we just went with AndyMark 4" HiGrips, live axle on ball bearings and 0.5" steel axles. Probably way overkill, but that drivetrain is pretty much indestructible.
https://drive.google.com/file/d/0B8v...it?usp=sharing
https://drive.google.com/file/d/0B8v...it?usp=sharing
Though I daresay yours is more compact. What does the rest of your drivetrain look like?