Potential for flex due to springs? Nonsense. Are you confusing that with what shims do? P67 lenses have a precisely machined bayonet attachment. It's quite solid. But I've only been shooting that system half a century; so what would I know?
You get a supplementary lens mount collar when you arrive at 300 EDIF focal length. I like to unitize both the lens collar attachment and the camera body thread itself together onto machined bar, which I laminated out of phenolic and epoxy-impregnated maple hardwood; and then in turn, mount that bar DIRECTLY to the platform top of my big Ries wooden tripod, or else a particularly solid CF equivalent. But that has nothing to do with any alleged "springiness" in between the lens itself and the camera body, but is simply to stablize the cumulative torque vector of that big long lens in relation to any potential support system flex. I've very thoroughly tested every aspect of this issue, and get very consistent results unless I've just done a sloppy job focussing itself (which sometimes does inevitably occur during the rush of bad weather or low-light shooting).
Shutter vibration? My older brother once sold Rollei and Linhof gear. He'd demonstrate the Rollei SL66 shutter by setting the camera on a table, and then a dime on end atop the camera, and tripping the cable release - the dime din't even tip over. If you tried that with a Pentax 6X7, the dime would land somewhere in the next county, and the Richter shock wave would topple brick chimneys in a six block radius. But that's due to the mirror slap, not the shutter itself. The mirror lockup features solves that. From about 1/60 and faster speeds, the shutter does its thing before the mirror hits anyway; so it's non issue.
And one more thing to set straight : I often shoot a 300EDIF at lower speeds. I have successfully hand shot it at high speeds, resting on a jacket atop a fence post or car roof sniper-rifle style. But a monopod? - absurd. These aren't like DLSR or even 6X6 teles. They bigger and heavier, and deserve solid support. And of course, the 300EDIF and 400EDIF were popular with astro photographers, who had their own serious definition of support.
I cut my teeth printing very precise big Cibachrome prints, so I think I know the meaning of sharpness. And although I prefer printing large format shots, I certainly do know how to make the most out of MF too, and especially the P67 system. And for that reason, which includes decades of experience, I simply can't subscribe to the claims posted by Light Capture. Making comparisons with Leica issues, or the well-known problem of the Nikon F-series with uber-long ultra-teles simply doesn't factor. Here we're dealing with an especially solid 6X7 SLR system with plenty of mass; but that in turn mandates a tripod support system with sufficient mass and rigidity.