How good is Blast Burn?

This is important to be considered, however since Char is not legendary is much cheaper to get and power up, plus two bars sometimes make the difference, specially since Moltres (at least in my battles) tends to die before launching the second Overheat

GP's DPS/TDO rankings say it's the highest DPS (as did the preliminary calculations posted on TSR), a bit above Moltres. In reality they will often hit the same breakpoints and have pretty much identical damage, but Moltres will have significantly better TDO.