Removing fines with a sieve is relatively easy to do, but is the resulting coffee clearly preferred by the majority of people ? If not, is there a way to quantify what percentage (or range) of fines is desirable?

Change? Yes

Define improve

As defined in the OP, "preferred by the majority of people".

Would you consider reframing your question to something like

"What flavor changes do people tend to find with sieving grinds, for either espresso or for brewed coffee?"

"Flavor" here in the meaning that includes the full range of sensory aspects, not just taste.

No

That's why 1Zpresso offers the JE and the JX grinder. Same grinder, different burrs. The JE allows more fines for a creamy sweet flavor. The JX has less fines for more brightness and clarity. Ironically the JE actually costs more.

So it always goes back to "how does it taste?"

P.S. I prefer the sweetness of the JE which has more fines

I didn't find that sieving my coffee made it better. Just different. Less rich. BUT, I really didn't experiment with changing my brew and grind methods, so I don't think I gave it a fair shake.

The problem with such simple questions is that it is a single parameter in what is a multi-parameter process.

I could say making the grind finer / coarser on my drip coffee improves / worsens the taste.
All of which are true.

Now if we throw in changing the grind, the filter, the coffee, the coffee weight, the amount, type and temperature of the water we have 128 combinations for the removing fines parameter.

cafeIKE wrote:The problem with such simple questions is that it is a single parameter in what is a multi-parameter process.

How about adjusting the amount of coffee to match the same TDS? This way amount of fines would be the main parameter compared. If fines are removed it would also result in quicker extraction, which would make the coffee taste weaker and would impact judgment, so some kind of adjustment is needed.

I think there's value in trying to simplify the discussion in order to reach pragmatic conclusions (when they exist, and I'm not sure that for the subject of fines they do). For example, when deciding between two very different kinds of beans (brewed using the same method), most people can decide which one they prefer. They don't need to be able to articulate the specific differences to pick a "winner", nor score them with a cupping card. A good example of this simplified approach can be seen in James Hoffmann trying to find the answer to "Which One Tastes Best?" (yes, when comparing instant coffee, but the same approach can be applied the question in the OP).

No offense intended, but the topic question is about as answerable as "How Long is a Piece of String?"
"Improve" is a loaded term. We drink drip here on Sundays. A new coffee every few weeks. Sometimes it takes a couple of weeks to adjust some of the aforementioned parameters so we have agreement on a good cup o' Joe. Some weeks the remaining coffee is DTB [Direct to Bin].

Hoffman is a talking head in love with the sound of his own voice. -1 for mentioning him
Gimme a chart, the reasons why and save me the idiotic adverts and a quarter hour.

I'll give my 2 cents.

This is base on my experience in filter coffee.

Removing fines does not always improve flavour. However it'll change the flavour.

Fines extracts faster for the same amount of coffee (More surface area)
It slows down the pour-over brewing time (More clogging)

Overall you end up with higher extraction. It really depends on your tastebud if you prefer under/ideal/over extraction.
Taste is pretty subjective.
It is like saying if medium rare steak tastes better than medium or well-done.
It really depends on the beans that you have (Or the steak in the above) and your preference.

I don't really like to sieve coffee. I think it is extra work that can be replaced by different grinders and burrs combination.

I also try different paper filters and different brewers to change the extraction process.

I hope this helps!