I'll agree with Soljin for the most part. Currently i'm sitting in Mostly WH gear (1900 aim (with stim), 1250 expertise)
I can pump out very decent DPS in Pyro, but only if i'm trying quite hard. I played PuG RWZ's yesterday and we were dominating. Merc Pyro still suffers however due to the fact that our survivability is nerfed, and our ability to kite is considerably borked. However, with a competent team to back you up, not all is lost. Play with a competent set of players, bring healers and tanks and don't be foolish in thinking you can ball carry or node defend by yourself
Just go with the pack, and hope to god you get healed. Otherwise, yeah, reroll, or learn to pillar hump... alot.
And as T-Assassin would say "be sneaky", because if you're standing out in the open, you'll die, fast. If you stand away, at range, and don't draw too much attention, you'll do great.