The reason for tightening a diff on a mini is to reduce the dredded 'diffing out' on corner exit where the inside tire gets all the power and just spins, so your corner exit is slower. A spool would change the handling huge i would think. I get my mini to handle well, and adjust the ball diff rather tight so it wont 'diff out' on corner exit. I run the M03r alot tighter than the M04 as the 04 doesnt roll over on exit and diff out as much. Always think of what you trying to achieve when working on your diffs, and consider what the drawbacks are. A spool id guess would be hard to get a nice corner entry especially on carpet, and im sure it would be very hard on the drivetrain.

Also consider that on really low grip situations with a tight track, sometimes a loose diff will work better also, allowing you to hold a tighter line. Just wanted to throw in these suggestions, its not like any single one is the ticket