This may seem petty, however on another forum someone asked if a particular Shimano chain makes shifting a whole lot smoother. Hypothetically speaking will swapping the OEM chain with a better quality chain make a difference in shifting? Would it be noticeable on a hybrid?

Negligible. Technique and maintenance influence shifting more.

Would you say the same for like a cassette? Not meaning to hijack this thread, but had noticed when shopping for future cassettes, Shimano has several 9 speeds that all appear similar hyperglides but different prices. Is the only real difference weight?

More often than not, the low end and high end cogs share the same cutouts, gates, ramps, and tooth profiles that make modern shifting so effortless.

However, I've found the cogs on the nicer cassettes deflect less because of the stiffer spider construction. This deflection is more evident on the larger cogs. They also have slicker plating that helps corrosion resistance and lowers friction.

The nicer chains also have slick finishes that combined with slick cogs and rings do make a theoretical difference in shifting slickness. Wether one can feel it or not is a different matter, and I doubt if I could feel a difference with one uncoated drivetrain part

Smooth shifting is a cumulative effect of numerous items, and a lot of them are absent in low end gear due to cost or artificial market segmentation. As you go uptier, the features start trickling in. But not all features contribute to smooth shifting, XTR ti cogs for example only cut weight over its XT equivalent. Sometimes the weight savings in cassettes is something like a steel lockring vs an Al one. Hold on to your Al lockrings to use on cheaper cassettes

Thanks for the info!

back to chains.....I like my bike to be as quiet as possible. Not that it's noisy but I can hear the chain.
Would a high end chain be quieter?

Is that a KMC chain? Shimano is quieter