At first I though the banana was utter trash but after cruching the numbers I think it's viable.

It heals 200 instead of Sandvich 300, and it have 10 seconds cooldown.  Since you'll probably eat when you are below half health usually 200 may be enough, and the low cooldown means you always have available. I definitely ran in situations when playing solo Heavy where I had sandvich on cooldow and would love a second bite.

Also, I'll probably be running Heavy during the Pyro update, taking advantages of all the players trying to outsmart bullet while dispensing banana-fueled justice.