Richard & The List-
Mixed tubing is often hard to verify, let alone have enough sister bikes of the same size & geometry to compare to, that said I have made some observations over the decades:
The tubing decal may well be not telling the truth, sometimes discovered by maintenance, Never seen a seamed 531 head tube before... Remove the paint and find different mfg. Stamps on the actual tubes than what it is supposed to be. Did it make a difference? The geometry I think was more important.
Things I can comment on with some degree of comparison, I have a few Masi GC's same basic size, different tubing, Reynolds, Columbus, Mixed. Fork rake makes a difference, The fork crown does also, the one with the twin plate crown is much more limber over bumps than the box crown bikes. In an experiment I even switched forks around, same conclusions. Side to side rigidity? No difference really noticed, 30 years ago the bike shop belief was that Columbus fork blades were stiffer laterally.
One thing does stand out from years ago, got to ride one of those "new" concept Klein bikes in 1975, now that bike was very stiff laterally, even amazing in comparison to any steel bike I had ridden before. Huge downtube and chainstays did make a difference.
One other observation, had a Teledyne Titan for a while, never could understand (other than aesthetics) why they raked the fork at the lower half, when braking all the flex was adjacent to the crown, work hardening at work, but they did not exploit it correctly.
John Jorgensen Palos Verdes Ca
Thinking of building a new OS tubing replica of my LeJeune.