Metabones 部分在這網站測試文章有Metabones RD 留言說明

Speed Booster ULTRA 0.71x 用 E-M1 Mark II 沒問題
Speed Booster XL 0.64x 部分，E-M1 Mark II 空間較小，XL 雖可接上使用，但可能碰到快門遮蓋甚至快門，無法使用

"I work on firmware development and I wish to apologize on behalf of Metabones for the confusion caused. In a nutshell, E-M1 Mark II is supported with Speed Booster ULTRA 0.71x. One of our technical support staff misread our (internal) measurement report with XL 0.64x and thought it applied to ULTRA as well (it didn’t) and based on that gave incorrect information to a number of our patrons. We got hold of the E-M1 Mark II and ran some tests with Speed Booster ULTRA and the combination worked all fine.
XL was another story. We measured and found E-M1 Mark II did not have enough clearance for the protruding rear element of the XL. It would appear to fit and work but the rear element might be touching the shutter cover or even the shutter blades themselves. So, E-M1 Mark II works fine with ULTRA 0.71x but not XL 0.64x. Hope this clears things up."

Evaluating the MTF Improvements of Speed Booster Focal Reducers
Jim Chung's Ramblings
https://jimchungblog.wordpress.com/2016/12/29/first-blog-post/