I'd suggest going ahead with an XM-L (neutral white T6 3C - see International Outdoor - probably want 20mm ) and run it like this one - direct drive single mode. I think Old Lumens did a mod similar to this one from those same lights - would have to search for it. Wait! Here it is! I see O-L beefed up the heatsink. I also saw in your linked thread that dchomak modded the heatsink with pennies.
You could go with the Nichia 219 as well for better CRI, but it won't be nearly as bright.
-Garry