How can Harry use Hellfire? I thought that was a trait only those in possession of a Coin could use?
I mean, According to WoJ, the difference between having Soulfire and Hellfire is "The kind of angel you are"
So, if the Soulfire Harry has is more because Uriel just sorta 'showed him the way' then a straight-up gift, Harry going all Black-Hat like the Fallen could maybe give him Hellfire.
On the other hand, if it being Soulfire flavoured instead of Hellfire flavoured is because of the Angel giving it, making Uriel Fall should cause Harry to get Hellfire instead.
Third Option would obviously be Harry patching things up with his disgruntled ex Lasciel
In either Case, I'm not sure I see much Benefit, since Soulfire can do the same thing as Hellfire, just with more then purely destructive spells