I will add more here later, but I've had hero damage up to 130K by LVL 25 in one game.
I've played at least a dozen rounds with this build.

At the end of the build you can have the potential to wipe out an entire team.
If combos are properly chained with this build, you can wipe out an entire team.
Think - 30%+ ability power if you've stacked and cast Arcane Power properly.

TIER 1 - the only viable options are Fel Infusion or Mana Addict, lately I've been choosing Mana Addict as it gives that extra kick necessary to dish out more combos.

TIER 2 - if you can avoid dying, Gathering Power is truly the best choice for constant additional damage all game.

TIER 3, 5, 6 - these all work together, you need them all or you'll be wasting your time.

TIER 4 - haven't even tried Phoenix to be honest. Although Pyroblast has a slow cast time, if you hide from the bushes to cast it on full stack w/ Verdant Spheres you're pretty much guaranteed a kill on heroes at or around 50% hp without concerns. Throw in Arcane Power and you got some serious one hit kills.

The strategy - once you've got ignite you become a force to be reckoned with.

Your goal is to kite as much as possible, while they're chasing you it's advantageous to throw down a Verdant Spheres w/ Flamestrike Combo. If placed right, they will always about face, but before they can get too far throw an additional living bomb on them - or first throw a gravity lapse then a living bomb.

The best part about when they chase, is typically you're running from the lead guy in front of their team w/ the whole crew following right behind (so I ignore the gravity lapse) - cause if you land the Verdant Spheres w/ Flamestrike in this build, it will chain and devastate their team, especially if you get off a Living Bomb at the tail end.

Honestly, gravity lapse is just a toy in this build, there's very little reason to ever waste Verdant Spheres with it. Here are some good combos and uses:

TEAM FIGHT STRATEGY
1. Verdant Spheres -> Flamestrike on the largest mass of people and then Gravity Lapse or Living Bomb depending on the circumstances

SOLO FIGHT
1. Verdant Spheres -> Living Bomb -> Gravity Lapse -> Flamestrike -> Living Bomb

If they're not dead, you could just send out a Pyroblast after, cause they will already be running.

All I can say if by LVL 20, if you're not devastating the team fights, then don't pick Arcane Power, but if you know what you're doing, there is NO REASON to ever get BOLT OF THE STORM, because nobody will want to go near you or they'll be dying more often than you that you'll have the chance to go heal. I've had moments where I've killed two heroes solo.