I finally got to test the 25 year old rubber after boosting. The speed is medium.
The booster actually did seem to restore the rubber elasticity maybe 80% of the way. I was able to hit some pretty good loops. The throw angle seems better, and the arc seems better.
It still seems like it doesn't arc quite as much as you would expect a new rubber to. However, I did have the rubber on my least dwelly outer-carbon blade. Maybe testing the rubber on a flexier blade would help. Also I wonder if baby oil would help the rubber add some dwell time.