The dampening effect doesn't come from the extension itself; but from the rubber mounts for the steel fan. It's doubtful whether the factory intended it that way, especially since they later went to a plastic fan.
I do believe a dampener does help longevity, if you plan to exceed 5000 rpm continuously. But I've done perhaps 100,000 miles in my TR3A with no fan or extension and the original crank has held up fine. OTOH my relatively stock engine won't pull over 5K in top gear, so I've never run it up there for very long.
The aluminum flywheel seemed to work out well too; though others report problems with keeping them on the crank when racing. Unlike some American engines, Triumphs are all "internally balanced", meaning the crank is balanced without the clutch, flywheel, front hub, etc. and then all those components should be balanced separately. On top of that, the factory didn't worry overly much about balancing them, and since replacement pistons are much lighter than the stock ones, it likely needs balancing anyway.
But I did notice that it took more care to launch the car with the lightweight flywheel. Not hard especially, but harder than before (when I could basically just take my foot off the clutch and let it chug away) to not kill the engine.