I stand corrected then. Regardless I think provided you can solder the LED directly onto it it should provide a substantial cut back in thermal resistance due to the removal of the dielectric bottleneck.
The “various other stores” were Intl-outdoor (no longer sold) but it turns out this would not work with them anyway… Intl-Outdoor's "Copper" XM-L MCPCB, I’ll get back to you once I find the “other” stores I was talking about. 