I have two A/B tests with results on the opposite ends of the spectrum:
First, Schoeps came out with a FET transformerless replacement body for the French M221, called the CMT 30 F. It was designed to use with existing M221 capsules, plus offered a new capsule in mylar. I compared an M221b with a CMT 30 FNU using 2 different M934b capsules, and found them to be so similar that I could not tell the difference. I couldn't hear the transformer, the tube, or the nickel diaphragm. I came to the conclusion (shared by many) that the capsule was mainly responsible for the sound.
Armed with this new info, I set out to create a mic sounding like a Wunder CM7/GT M7, using the same Thiersch M7 transducer, but in lollipop form. I tried it on a Violet Global Pre, a Blue Bottle Rocket Stage 1, and an AKG C480B. None of these came anywhere close to the sound of the Wunder, particularly the Blue. My conclusion was that in the U47's case, the capsule provided only a small part of the sound.
Lastly, I have used the Millennia STT-1, which has the only transformer in/out circuit that I know of, and in that case, the transformer made a much more significant change in sound than in switching from tubes to solid state.
Stephen