Do fresnel lights produce an equally even light in spot mode as in flood mode? If not, what is a typical drop-off ratio?

This will vary from different models and manufacturers. The best thing to do is play with the individual lights in a dark studio and see what looks you get depending on the zoom amount.

As per michael. Also if you do find that the beam is uneven, especilly with open faced units, a light frost, or light diffusion can help smooth things out.