It is actually 0.64x, not 0.67x :-)
"The new Speed Booster XL 0.64x reduces the full-frame crop factor of the Panasonic GH4 from 2.0x to 1.28x, thus effectively transforming these cameras into APS-H format."
"For example, a 50mm f/1.2 becomes a 32mm f/0.80"
http://www.metabones.com/products/details/mb_spef-m43-bt3
http://www.metabones.com/assets/a/stories/Metabones_Speed_Booster_XL_0.64x_Press_Release_5Jun2015.pdf
Stab math is correct.