The lens does not become dimmer if you put it on a smaller camera. Ignore all equivalence unless you actually own both formats of cameras. In that case, equivalence can help you imagine what a lens rendering (FoV, DoF) might look like on the other format.
Also, the 100mm macro gives 1:1 magnification on FF, so on APSC it appears to give more than 1:1 magnification. If you go into the equivalence vortex you will just get confused over useless things. The DFA 100mm is a great lens, on ASPC and on FF. You will enjoy it on either camera, or both.
Originally posted by Joe M 1.5 x F/2.8=F/4.2 or does that not apply to this lens?
You cannot just multiply 2.8 by 1.5x. Even hardcore equivalists only multiply the EV, not just the aperture number.