Hmm, QUICK TO THE BRINK-MOBILE!!! DUNUNUNUNU!!!!
Anyways, I think Heavy's will be great medics and support because of, well, this, they can take alot of damage and they will force the team to slow down and not butt rush towards the objective.
Or, they can let the team rush the objective and mop up the mess the team created when they arrive. Think of it as "Oh, the reinforcements have arrived!"
Heavy Medics will be highly successful so long as they are close to the fight. Otherwise, they would be a waste of manpower and valuable resources if they are nowhere near the fight and can't get there quickly.