No experience with a 4K projector, but just doing some math. With a 4K projector, I believe you could upscale 1980 * 817 content to 4,096 * 1,743 to be displayed on the screen (assuming you had the right scaler).
With a 1920 * 1080 projector, you get roughly 32% more pixels by scaling/using anamorphic lens than by just zooming (i.e. 2 million pixels vs. 1.5 million). That is a benefit that is not easily ignored.
However, with a 4K projector, I think the issuing of scaling just to 2.35 and zooming vs. scaling to 16:9 and using the lens is much less of a benefit for the lens option. Here's what I mean. With scaling to 4,096 * 1,743, that is 7.1 million pixels, which is roughly 455% more than the original content.
Sure, you can keep scaling to fill up the whole panel, which is 8.9 million pixels (4,096 * 2,180) or 569% more than the content.
I know scaling has benefits, but is there really that much more to be gained by having 569% more than the content vs 455% more than the original content. Even if there is some small benefit, I can't imagine it would be worth the other issues that might come into play with having a lens in there.
So assuming the light output is okay, it sure seems that with all the extra resolution available with a 4K projector, is it really necessary to fill out those last pixels?