Theoretically, yes. However, you would be pushing a lot of current through it, a lot more than say your XPL HIs since they have a lower forward voltage, especially the 80CRI one.
That means you have to beef up your connections to handle that massive current.
Other than that, I would calculate the amount of lumens you could get out of the R70 LH351D to be about:
12k lumens = 1000 lumen per emitter
1000 lumens x 1,10 efficiency factor of optics = 1100 LED lumens needed to reach 12 000 lumens with optics
1100 lumens/emitter = 12,6W of power
12,6W of power XPL HI= 1100 lumens XPL HI = 1450 lumens Samsung LH351D R70
1450 lumens LH351D x 12 = 17400 lumens
17400 LED lumens = 15 800 total lumens
So you would get 15 800 lumens out of it, a 31% boost without doing anything.
HOWEVER, since the forward voltage of the R70 LH351D is lower than the XPLHI, and even lower in the case of the R80 LH351D. Meaning you would push more current out of it, about 600mA/emitter. Meaning an additionnal 7,2A.
This is what you would get with a FET:
3,6V Forward V XPL-HI = 1100 lumens XPL HI
3,6V Forward V R70 LH351D = 1600 lumens
1600 lumens x 12 = 19200 lumens.
The 80CRI version can push even more current, and with 4 cells, you have more headroom to push even more power to get to say 21k lumens as you said.