Idly thinking about an EF speedbooster

[kye]

I'm idly contemplating buying a Metabones Ultra 0.64x speed booster for my GH7.  This would take me into the world of EF for the first time.

I'm completely familiar with speed boosting and crop factors and all that jazz, with years of experience from my 0.71x M42 to MFT speed booster and (many) M42 lenses.

What's the deal with speed boosting to EF?  
Is the Ultra 0.64x worth it over the normal 0.71x adapter? (they seem to be similarly priced used).
Is there a different one I should consider (other than Metabones)?

Essentially I'd be getting it to shoot shallow DOF (like I do with my M42 Takumar 50mm F1.4, etc) but with more modern / cleaner results as M42 lenses are quite vintage and far dirtier than fast EF glass, especially when shooting wide open.  AF is of little importance to me, so I'd be expecting manual focus.