Ok, well a couple points. There's a reason people aren't putting points into dex, person: the sage ring +2 gives 40 virtual dexterity. The cap on cast speed from dex is 50. That means if one has even 10 dex and a sage+2 they'll be capped out.
Second point. Black serpent: it isn't good. I would say, and this is from watching good pyros, that the 3 most important spells a person could have is vestiges (I guess if one is especially good at free aim chaos fireball could be superior with the lingering lava), black flame, and fire surge (a "poke" spell). And as far as spell "combos", the unfortunate (or fortunate) truth is that successful pyro still involves a whole lot of melee, cause spells aren't quick or unpredictable enough that they aren't going to whiff 95% of the time without the addition of melee to mix things up.