Digital Sound Factory Hammond B3 - Leslie effect doesn't speed up or slow down

I just bought the Hammond B3 plug-in for Sonar. I was particularly looking forward to having a decent B3 where the Leslie effect speeds up and slows down in the way a real one does. The product page on the DSF site even refers to the ability of the original instrument to "create a wonderful doppler vibrato effect, especially when the speed is changing". I've installed the instrument and the sounds are really usable, except that the Leslie seems to switch from being off to being on. without the speeding up effect. Same when switching it off. I've tried assigning it to a midi hardware slider to see if it transitions slowly. No luck. I move the slider up and around the middle, the effect switches from being completely off to being full on. Has anyone managed to work out how to make the effect perform more realistically? Alternatively, does anyone know of a decent plug-in Leslie effect?

There are a couple of freebies. Mr. McDonald and Spinner LE. Used them in the old days and they can be quite convincing in a mix. They are only 32 bit plugs, though. They do work in Sonar.