Does lifting the keel going downwind makes noticeable difference?

Remember, the question of lifting the keel was specific to DDW sailing. In a modern planing boat, you would avoid sailing DDW, if you have the leeway. Your VMG is generally better sailing a tacking course off the wind, but that doesn't address the question.

This is a very interesting idea and I feel there is some real value to be gleaned from experimenting with this.

 

No, it won't!

Even for a given amount of lift, the induced drag goes down as the speed increases. Combine that with a decrease in lift and a keel's induced drag becomes negligible.

To be clear: I'm answering "No" to the quoted comment. My answer to the original question would be "Yes, generally speaking",

 

The answers are (IMHO) no, yes, and perhaps..... depending on the situation.

Yes, the difference can be noticeable if you are already sailing well enough to be doing well in championships and are up against comparable boats.
No, the difference in speed is not normally noticeable if you are sailing by yourself and you're not a very good sailor with very good VMG information.
Perhaps... if you're somewhere in between the two situations.

The point is that no one can answer the question unless it's answered in more detail.

I'm not sure that you'd always want the max keel down in a sportsboat; plenty of high speed classes raise the centreboard downwind at high speed to prevent the boat from "tripping" over the 'board. In long windsurfers you can actually feel the centreboard's lift trying to flip the whole windsurfer over when going downwind at high speed and the same sort of hydro lift and other factors must be happening in other types. In Formula 18 cats, for example, lifting the centreboards up is standard procedure downwind as it's generally felt to allow the boat to slip sideways under gusts rather than slide sideways when hit by gusts.

 

If you can maintain sail/ hull balance, and stability, like on a cb longboard, raising the cb all the way in bigger winds is like being shot out of a cannon.

(See if CT249 agrees.)

The problem is the center of effort of non windsurfers, or non kite sailers doesn’t move much. And as far as I can tell, most rules forbid it. (Although watching Stradivarius sail by in 6 K of TW, downwind of Cattle Passage, with her rig canted was a kind of a zen of balanced nervousness.) A pity, since moving/canting a rig can act as a kind of automatic reefing, as far as projected area. Although, in a jib/ mainsail rig, furling the jib downwind while leaving the main full up, can do the same kind of thing, if you’re planing enough to move the center of lateral resistance aft. You can even pull up the centerboard a bit to control CLR, so you have something to rotate on.

 

The crappy part is that if you don't need all the RM, you probably do want all the foil area to reduce induced drag (all sail flying and low wind speed). And if you don't need all the foil area, you probably do want all the RM for best performance (sail reduced and high wind and seas). On most sport boats, you want to add sail to consume all available RM at the design heel angle.