explains how the Excell +1 focal reducer is not a competitor to the Metabones Speedbooster and how they are different:
A lot of people looked at the Excell +1 last week as an equivalent to the MetaBones SpeedBooster, which is an amazing product. The Excell +1, however, is not a competitor for the SpeedBooster. We wanted to create a focal reducer that could do a lot of the same things, but at a much more manageable price. This video gives you a better understanding of the two different products, as well as a few more technical details and examples.
The main takeway is that the Excell +1 is only meant to be used at apertures f/2.8 and higher, as can be seen from the following images:
Here’s a video comparison:
What do I think? Even at f/8, the Speedbooster far outclasses the Fotodiox Pro Excell +1. Cannot recommend it, for $160. Bohus explains in the above video that those who purchased the Excell +1 and were expecting the Speedbooster can return it for a full refund, while those who wish to keep it can get a $50 credit on their next purchase.