can't really be any help on the three phase/vfd thing.     we did have one barn that was using the single supply double feed line configuration.  that's since been switched to a double auger/tandem setup.
 without having any real numbers in front of me, and going by guess and by golly, it seems like another feed line vs the vfd would run about the same cabbage.   but it would certainly be handy to be able to control supply auger speed.

interestingly enough this was topic on a trip we partook of last week, anecdotal evidence would indicate that a particular motor brand purchased from one entity may be built to different specs than a similar branded motor purchased from another supplier.???????