Except if he plays that well you will really want that age 31 year under team control but now you've given that away. It seems unlikely all 3 of Mitchell, Wiemer, and Frelick make the OD roster. The Brewers can earn an extra draft pick with Frelick, they can't with Wiemer. This really seems like a no brainer.
As for the competitiveness, even if Wiemer is a 5 win player this year(highly unlikely), two weeks likely wouldn't make a noticeable impact(1 win at the absolute most). The potential of 1 extra win is not nearly enough for me to throw away a year of his prime