I've got to disagree simply from the point that I've seen the difference between having it mounted on 2-seperate tripods. To me the problem is the mounting of the body allowing a huge weight of the lens/converter to hang over the front screwing up the balance. Placing the body/lens on a rail will allow me to shift the balance backwards. For the life of me I simply can't understand why a tripod mount wasn't designed for either the converter or the lens.
Having done a few shots recently with the Mamiya 300/2.8 + 2X converter on the ALPA FPS, I would definitely concur with this.
There is no doubt that these big shutters have the potential to cause vibration, but the key thing is how you damp that vibration. You can't just leave it up to the camera - basic physics says that there's kinetic energy created by the shutter, and that has to be dissipated somehow.
There are two things you need to take care of.
Firstly, make sure that the kinetic energy has little chance to create a torsional force - this means get that camera and lens as rigidly mounted as you possibly can.
Once you've done that, then ideally you need to increase the mass of the entire system as much as is feasibly possible. The heavier the entire rigid system (camera, lens, rail, tripod mount, tripod, etc etc), the less effect the dissipation of the kinetic energy of the shutter will have.
Kind regards,
Gerald.