He's no better than Mark Ingram. Give Ingram 321 carries and the numbers would compare.
I'd like to see him put up those numbers against teams that can't load the box because Antonio Brown is on the field.
You're comparing two backs who are completely different. Ingram has never been able to shed tackles and do the stuff Bell can. We were lucky last year to have Kamara who saved us a number of times when Ingram couldn't show up in games (see Rams game and both playoff games). A better comparison to Bell is Kamara.
Pittsburgh's offensive woes had nothing to do with Bell last year. Their offense has always underachieved in the past decade despite who the RB is. They have a mediocre coach who has cruised through his tenure with high end talent and don't have much to show for it outside of that one SB.