The AL Gold Glove was for LF and he is probably among the best there. Agree he is probably an average CF now. Pretty good instincts. His speed is good, though merely around average for a CF as there are very few CF with below average speed. I think he is fine there. He'll cut down the handful of mistakes like most young fielders do.
We just don't know if he can hit that well. There is so much movement in his setup that the coaches probably can settle that down and speed up his bat a bit. But will he ever have the batspeed to hit a good fastball? Not sure. There is probably more power in him though. Could easily see him hitting 20 HRs.
Long is a good coach, but he is also no miracle worker. Hopefully he can connect with Marsh and smooth out his issues a bit. But we also should not expect Long to make batters be something they are not. It is very possible that Marsh and Vierling are destined to be platoon partners. Like it or not we traded for Marsh to improve what we had (which he did), but there is no guarantee that Rojas won't take the job by the end of next year. Rojas offers even better defense and speed, though also a lower floor on offense.