Gylph of Storm
The thing is there isn’t much else you can pick if you want some extra damage. Signet of Fire, Glyph of Storms and Ice Bow are the things you usually take along for some extra burst.
And the idea is to cast the Glyph (or anything really) when your Font is on CD. So comparing their DPS values doesn’t say much, they just add up. So it’s not a DPS loss in a Staff Tempest build, though the Lightning Storm does more damage and adds vulnerability, yes.
because you use it when you would be using fireball
you can’t spend all of your time casting lava font. it has a cooldown.
so yes, you are quite incorrect