Andrew, When you get a chance, take a look at Falk Lumo's discussion of camera equivalence. A 25mm f1.4 lens on a micro 43rds sensor is equivalent to (ie., indistinguishable from) a 33mm f2.0 lens on an APS-C sensor or a 50mm f2.8 lens on a full frame sensor. The nice thing about the speed booster is that it lets you use a full frame lens on an aps-c sensor in a way that makes it equivalent to the same lens on a full frame sensor, but this is only true if you shoot it at the equivalent aperture. So if you are shooting a 24mm at f1.4 on a full frame sensor, you need to shoot the 24mm lens with speed booster at f1.0 on an aps-c sensor to achieve equivalence.
Noirist, I'm still trying to get my head around it but I don't think this normal (and normally correct, by the way) logic applies to the Speed Booster. The equivalence that you mentioned is applicable when mounting a full-frame lens onto a normal, "dumb" adapter, to use it on a S35 sensor. In this case, the image circle that the lens projects, is the same size as it would be on a full-frame sensor, far bigger than it needs to be to cover the smaller S35 sensor. A lot of this wider image circle falls outside the sensor area and is basically wasted, with just the central portion being recorded. For this reason, the brightness is the same but the apparent DOF is greater as you're basically cropping into the same image with a smaller frame; not dissimilarly to if you cropped the image in post (though obviously without the decrease in resolution etc).
The Speed Booster is a whole different kettle of fish. The optics that are built into it actually focus the larger image circle down to the smaller S35 sensor. This means that the image is actually brighter as the light is concentrated (think of burning holes in things with a magnifying glass in science lessons!). Because the same image circle is captured by the S35 sensor as would be captured by a full-frame sensor, the DOF is actually the same as on full frame.
Therefore, when using the Speed Booster, using that 24mm f/1.4 that you referred to would give you baically the same image as using a 24mm f/1.4 on full-frame (same apparent depth of field) but the image would be one stop brighter due to the concentrating effect of the optics.
If I've got my head round this thing, then I THINK that's all correct!