OL is right. I've said the same before and if you look at Matchs graphs of - for example - the XPG2 here:
Emitter Test Results pt4: XML2, XP-E2, XP-G2, and Nichia219! (along with sinkpad tests) (I’ll just paste the pic)
and then take a look at the graphs at 1A.. the difference is negligible. Maybe there is a 5% increase at such "low" currents if you use 3 LEDs. An XML at 3A gains about 10% on a copper Sinkpad.
So.. are copper boards cool? Yes. Are they the messiah everyone thinks they are? Not unless you drive your LEDs really hard (4.5A and up for a triple).
Maybe I would still use them just because they are cool, but on the other hand.. reflowing one LED is hard enough, I dont want to reflow 3 at once. :D