I read that the 6G72 is smoother than the 6G74. Presumably because of the shorter stroke.

How refined is the 6G74 meant to be? Mine is relatively smooth but it definitely is not silky smooth.

And how smooth is the 6G75 compared to the 6G74 especially when in a 3rd gen?

I have a 6G74 in the Ralliart and a 380. Ralliart motor a lot smoother, especially at high revs. 380 has better noise suppression though. Can't comment on standard 6G74.