I guess it depends how much you're willing to cherry pick.
29 for 334 and 4 tds in the first 8 games with one game missed. That prorates to 58/668/8. Meh.
28 for 324 and 4 tds in 6 games if you ignore the game missed and his 1/10. That prorates to 72/864/11. That would have made him te7.
Heath miller's best season was 71/816/8. His second best was 76/789/6. Those would have been good for te10 last year.
I can see getting excited about Roethlisberger, but rivers has actually been more productive and has targeted his tight ends a lot more than Ben.
So if we use the most positive cherry picking, ignore the injuries, and assume he will be more productive than the very good tight end Roethlisberger played with for most of his career, you might get something between te7 and te10. If you consider any of the injury history, changing teams, his fairly weak draft pedigree, his inability to earn a more significant role, and the fact his team was willing to let him go after hours rookie contract ended despite needing receivers and having an aging tight end, you could probably put him lower than that.
I'm just not seeing top 3 pick here.