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Amsterdem, I believe the focal length stays the same, merely you lose the crop factor, so the 50mm MP is actually a 50mm MP rather than a ~80mm MP and certainly not a 35mm MP. Or are you trying to say its like a 35mm MP was attached to your body, thereby making it 50mm again?
I'm afraid not.
Focal length is a physical property of the lens. The only way to alter focal length is with a teleconverter or telecompressor. The former increases focal length by a multiplier, and reduces the maximum aperture by the same multiplier (as the physical aperture remains the same). The latter does the opposite, focal length is reduced and maximum aperture is increased.
The Speed Booster is a Telecompressor combined with a mount adapter. It does indeed reduce focal length and increase lens maximum aperture. this is possible since aperture is simply the ratio of the focal length to the physical aperture and the former is decreasing, which decreases the ratio.
For example, a 50mm f2 lens has a physical aperture of 25mm, the speed booster multiplies the focal length by 0.71, giving a new focal length of 35.5mm. 35.5/25 is 1.478, so the new aperture of the lens is f1.48 (given the multiplier, you are getting slightly less than a stop because the ratio of APS-C to FF varies by a small amount depending on the specific sensor and Metabones picked a nice round number in the possible range but one that is slightly less than a full stop, also a 50mm on FF is not exactly the same FoV as a 35mm on APS-C).
Crop factors are merely a convenient short hand to match Field of View between 135 format SLR and DSLR's and the various smaller APS and 4/3" format sensors in use today. Your 50mm lens does not become a 75mm lens when mounted on a 1.5x crop body, it merely has the same Field of View on 1.5x crop as a 75mm lens has on a 35mm body.