a healed heavy doesn't need anything else to rip and tear pretty much everything. ubers are just for keeping the heavy alive if his hp go really low (->7? seconds invincible+full hp again after that). or, well, if you are almost dieing since you also shouldn't die.
of course you should uber before one of you dies :V
Ammo, the heavy is a great close quarters class' yeah the pyro or soldier get a significant boost while uber, those classes have gimmicks (the pyro needs to chase down his opponents in order to finish them off, the rockets can only be fired a certain amount of times per 7 seconds,where the heavy can just hold m1)
When the heavy is ubered because of his higher dps it makes it easier to finish of targets(in a given range)
Think about this, your on dirtbowl 3 and red is on the 2nd cp, what are you going to do run out there with pyro or run out with a heavy?