Wiemer had a high ceiling but it all depended on if he would hit enough. I agree Mitchell had the highest ceiling. I have him comparable to someone like Grady Sizemore, Wiemer closer to Hart/Jenkins and Frelick closer to someone like Kenny Lofton.
Frelick has the highest floor of all three OF's but being the weakest of the three defensively in the OF. Frelick should only be playing LF and CF. I do not like Frelick in RF at all. Mitchell and Wiemer are very close defensively in the OF. I still rank the three OF's: 1. Mitchell 2. Frelick 3. Wiemer.
I don't believe the Brewers can move either Wiemer or Mitchell in a trade and get anything of value back. Basically if you trade Wiemer now you are getting a AAAA player back at best. If you trade Mitchell now you probably don't get anything back in return. If you want to trade an OF and get some value back it would be Frelick and not Mitchell or Wiemer.