Hello all,
Alright more updates..
TL;DR: GXB172 works great for XHP35 (HI), driving it at 2.5A 14.7V. Power source was a single LG HG2 18650.
I finally got my hands on an XHP35 LED on a MCPCB (from Kaidomain), so I could do some real life testing of the GXB172 driver on 12V (4S die) LEDs. The candidate of choice for this test was a XHP35 HI, 3000K, C2 30G 80CRI LED. The light was mounted on a medium sized heatsink for testing and connected to my GXB172 in 12V mode.
For this test, the goal was to run the XHP35 HI at it's maximum output (before output starts to suffer).
I referred to Texas_Ace's excellent experiment here: https://budgetlightforum.com/t/-/43473. From this, I determined that 2.5A was the maximum one would want to drive an XHP35, after which you get no additional lumens and run the risk of damaging the LED.
Soldered up an XHP35 and connected it to my GXB172 dev platform... and turned it on.
... and... it works!
Note that based on my previous work with putting together an actual 17mm driver, the 17mm PCB performs at least as well as, if not a little better than my 'dev board'!
So I fully expect the GXB172 to work just as well, in the form-factor PCB.
Test was conducted at a variety of power levels all the way from the lowest setting (about 2-3mA @10.2V) to the LED, all the way up to 2.50A at 14.70V (36.75W to the LED). Battery input was measured at just over 40W, so about 90% total efficiency (including wires etc, probably a little better in a well-put-together flashlight), with higher efficiency (>95%) at more realistic power levels.
I plan to have this LED put into one of my old SK98 hosts since that has a zoomy lens. This should make for a pretty nice 'budget' throwy light, and pair nicely with my 144 Convoy S2 'floody' light!
So stay tuned for a 20mm version (since that needs a 20mm driver).
Thanks for following this project and as always, more to come soon 
!