Tre Mason is not an every down back. Stacy is good but not great. That's why i had the Rams taking Gurley in my mock. When you looked at their situation, he was the logical choice to best improve the offense. They now have Gurley as an every down playmaker and Tre Mason as a backup who can also make big plays.
I wouldn't be interested until after the draft because this draft is deep at RB. If the steelers don't draft a RB or it gets late and the good ones are gone, i'd be in favor of trading a late pick for Stacy.
He may not want to go to Pittsburgh though because he wants to play. He's probably OK with being a backup but not with being #3. STeelers could offer him the chance to beat out Deangelo Williams for #2 and of course, while Bell is suspended, Stacy would have an opportunity to play at least for a few weeks. That's likely better playing time than he'll get with the rams,