first to the korisu - if you washed them and they are still slick they aren't likely "official" Tornado balls
interested how your tumble works...let us know
the tumble is like a rock tumbler...lot of stuff written on doing a home tumble which i may find later - however, the old "pink" Tornado ball need refreshing often which is where the washing and all the tumble advice came from...the newer red 5 hr tumble lasts much much longer - i haven't heard yet of anyone trying to refresh these
how long can you tumble before the ball is too small? well, tumbling like we do, doesn't really take any size away...people of measured before and after with no loss...when you get a fresh pink ball the "fuzz" or "nap" is raised on the ball from original tumble...when the ball "loses" the fuzz I used to the fuzz left the ball but you never see it on your table and thats because the fuzz doesn't leave, it is just compacted back down on the ball
when you tumble, you are just raising the nap again so you don't impact the circumference of the ball...oh the slightest bit perhaps but even with real good tools, the change didn't register...so, it would take either a really long time and many tumbles or a super harsh unrecommended sand paper before you would begin actually filing off the outside and impacting the circumference
in this case, if your tumble experiment doesn't work, don't kill yourself, just buy some new balls and get the real deal
with that said, don't know where everyone is headed in your play group at the office but if control and long lasting are goals, i would try and get a hold of some Warrior balls...lot more control, even than the improved control of the 5 hr tumble...i would do this only if the players were unlikely to move to local weekly tournaments or regional and national ones - if they were, i might not do that because it will take a lot of adjustment if they play on Tornados to go from the lighter stickier Warrior to the heavier, denser, less stickie standard 5hr tumble