The derailleur doesn't dictate the pull ratio, the shifter does. Pretty sure the pull ratio is different for 10 vs 11.
The shifter also doesn't pull constant cable between each gear.
The derailleur is expecting: a certain cassette profile, and a certain amount of movement (diagonally) for every millimeter that the cable pulls. The shifter is expecting that the amount of cable pulled translates into the sprocket positions. When you mix brands (or generations), this is where stuff tends to go off the rails. A 0.1mm doesn't mean much by itself, but after 11 gears, it's off by 1mm. Additionally, the cassette profile thing isn't just an afterthought; when you need to bump the derailleur away from the cassette to clear a larger sprocket, it moves it away from ALL the gears...so shift speed/quality suffers there, too. We paid dearly for a switch from a double-sprung derailleur to a single spring, but most everybody wasn't even aware that it happened. Let's not even bring up the tangled mess of 10 speed road is different from 10 speed mountain, is different from
new 10 speed mountain, is different from
new 10 speed road.
It's moot, anyway.
Current generation Shimano 10/11 mountain, specifically the one that I linked to, appears to share all the key spacing points for using it on a 10 speed setup...their setup. Whether it works with an old 10 speed shifter, or another brand, is anyone's guess.
@Paul H would have to do his homework, because Shimano isn't interested in making sure their newly refreshed derailleur (compatible with 10 AND 11 speed) works with 8 year old stuff. Aside from the derailleur not officially being able to shift a 50t sprocket (Shimano is usually conservative there, but that's a BIG jump), one would also need to make sure that the spacing of the cassette that had a 50t ALSO matched the spacing that Shimano sells their approved cassette with. I guess you could use a Goatlink with the linked derailleur, but again, who knows how well that will work here.
In short: the 2t smaller chainring looks way more attractive when you need to figure all that out, usually by outlaying cash to do it. Raceface makes DM chainrings all the way down to 24t.
