My memory may be faulty, but I don't remember Harvin being the power runner that Corley is when the ball is in his hands. I remember Harvin's game being more about speed and elusiveness.
Byars may be a more apt comparison. I don't remember Byars all that well, because I think he was only with the Jets for a year or two towards the end of his career. I think he was definitely a powerful runner, but maybe not as fast as Allen. Byers was also a fullback and weighed around 257.
Byers was only here for 1 year before retirement (1998) he was a receiving fullback I think if you are thinking of a former Jet to compare to Braelon Allen, you can look towards LaMont Jordan. Big violent runner with some finishing speed especially early on
This is what I've thought of him throughout the draft process. He's already a better receiver and better in pass protection than Henry was out of college. Being the youngest guy in the draft who knows maybe he does bulk up to that size or close.
Here's a breakdown of Fashanu's play in the Commanders game. The guy making the review had Fashanu as the #1 OT prospect in the draft ahead of Joe Alt. I think he does a good job.