I think you pretty much nailed it with your statement.
"Yes, a standard crappy fibreglass PCB, when laid out correctly, is better than aluminium MCPCBs at pulling heat from LEDs.".
If the ALU dropin is assembled with a high level of attention to tolerances, heat sinking and quality components...
Re: FS: Arc LS Triple LED Modules
Are you using an AMC7135 driver? or custom. I ask in the case if i wanted to drive the LED's a bit harder
Nice drop-ins and fair prices