Through their first eight games...
Antonio Brown: 61 catches, 701 yards, 3 TD
Emmanuel Sanders: 37 catches, 494 yards, 2 TD
Jerricho Cotchery: 29 catches, 424 yards, 5 TD
Mike Wallace: 36 catches, 480 yards, TD
Antonio Brown and Emmanuel Sanders have more catches, more yards and more TDs through their first eight games than Mike Wallace does. Emmanuel Sanders and Jerricho Cotchery have more yards per catch. All three have more TDs. Clearly, Ben Roethlisberger makes his WRs better.
With that said, a tall, thick WR should help open the offense up even more.