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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A 300 Timur's Lash out classes nearly every current Vanguard gun at 331. It 1 shots thralls during the Crota fight, nearly 1 shots Acolytes, and just plain wrecks regardless.