Don't waste the motes of light, the weapons aren't updated to the standard damage of the expansion. Meaning they are still only 300/300 and not 331/331 max.
[b]Edit[/b]: Tried with Timurs Lash, just changed the upgrades and not actually upgrade the total damage.
[b]Edit v2[/b]: With the coming House of Wolves expansion, you will be able upgrade ANY Legendary gear, so that means Iron Banner weapons as well.
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Everyone crying about this is a retard. Only the weapons got stronger, not the enemies, so in the end it doesn't matter and you are all just wasting time crying like farm peasants children