In their sparring sessions, Marlon supposedly gave Thomas fits before getting his jaw broken.In a real 15 rounder at 147, does Marlon have what it takes to score an upset?

It's very possible, but I would put my money on Hearns.

Hearns would win.

I would go with Hearns & here's my thinking.
Starling was an under-rated boxer and he could possibly give Tommy fits. But he could never KO the Hitman.
Everyone knows that Hearns is an incredible puncher, but remember how great a boxer he was also?? Sparring is different than actually fighting, and I think Hearns's heart would make it possible for him to find a way to stop Starling late. I think at this level, one thing that has to be questioned is desire and I think Hearns loved boxing way more than Starling did. Even today, Starling seems bitter about boxing.
No "Starling Stomp" in this match up, but not the easiest win for Hearns either.