Hmm, a dedomed xpg2 on a copper board pulling just 3.5A is a dissappointment, my ideal current is 4A, but some want to squeeze 10% more out of it at 5A .
I don't have any of those, but as with the newer xml2 emitters, there is word that the newer xpg2 emitters have a higher voltage than the older ones, and blow at a lower current than before. You might have one of those.
A solution could be to get an older batch xpg2, I would expect Fasttech to have old stock (on aluminium boards or bare) although I would not directly trust their bin claims.