Using speed boosters / focal reducer on a crop body can have great results. You can turn your 50mm f1.8 lens into a 35mm f0.9 in seconds.
This video will explain what a speed booster / focal reducer does and how it can help you get the low light gathering ability of a full frame camera with a crop body.
The lens opening is determined by the opening of the iris inside of the lens, not by the front element.
In the tests, I should have compensated for the concentration in light with ISO. This explains why the speed booster setup gave a brighter image than other setups in these settings. The diagonal measurement is used to compare sensor size.
The focal reducer is placed between the lens and the sensor. It will take the light from a larger part of the lens than the crop sensor would use and compress it to fit a smaller sensor.