I didn't design the Druid spell set. The fact that the best single target damage spells - by an enormous margin - available to Druids are Cold-based is not my doing. This is true even on a Fire Druid. This is true even on your approach to a Fire Druid. Playing a "Fire Druid" doesn't mean you spitefully avoid casting your best spells because they're the 'wrong' element.
For the 17/3 method to work, you need some fairly specialized spells:
- If it scales at a cap lower than 20, it generally works better as pure because +1 caster level/max caster level > 6% critical + ~20 spellpower for a level 30 gear set.
- If it's a Druid spell with a 20+ maximum caster level, it works better as pure.
- If it's a non-Fire/Force/Light spell, it works better as pure.
So let's review the spells which match the criteria to justify 17/3:
- Arcane Pulse: There are at least 4 single target spells better than this that Druids already have so it's tough to find time to cast it in your rotation.
- EA SLAs: Useful for saving some SP, but generally weaker than existing Druid spells (and for single targets, weaker than existing Druid SLAs).
- Ruin/Greater Ruin: Do decent damage, but nothing significantly beyond what you could already do, cost two feats and have relatively long cooldowns limiting them.
- Hellball: Generally terrible spell, but you're going to do comparable damage either way because AoV only affects one of four damage types.
- Flame Strike: Approximately the same either way, but generally supplanted by Firestorm
- Produce Flame: Approximately the same either way, but generally too weak to use unless you take Master of the Wilds (in which case pure is better)
- Wall of Fire: Normally not used in epics due to low damage, but roughly equivalent either way.
- Flaming Sphere: No one uses this for damage.
- Splinterbolt: In theory, does more damage as pure. In practice, getting 1/6th of the way to the next damage step probably isn't going to be helpful.
- Energy Burst: About 4.5% better as 17/3, but with a cooldown
Everything else? Pure > 17/3. In that 'everything else' is virtually every high damage spell you'll actually be casting.
I broke down precisely where there would be an advantage for 17/3 over pure above. It's a short and mediocre list.
I think part of the problem is you just can't wrap your head around how important caster level is to a damage caster.
Let's say you cast Firestorm with 40% critical, 600 spellpower (from all sources in common), Scion of Fire, Scourge fully stacked.
On a 17/3 Druid, you'd be dealing 85.5 * (1 + .46 * 1.25) * (1 + 6.30) = 983 damage
On a pure Druid, you'd be dealing 103.5 * (1 + .4 * 1.25) * (1 + 6.1) = 1102.3 damage
Most of the remainder of the Druid fire/force/light spells - Body of the Sun, Sunbeam, Sunburst, Word of Balance, Fire Seeds, Fire Trap - have the same sort of pure advantage. All non-fire/force/light spells do better as pure since AoV doesn't improve them at all.
Empyrean Magic doesn't really have any more 'synergy' with 17/3 than with pure since it's relatively easy to stack and maintain.
Basically, by taking those three levels of AoV you're throwing away more than you're getting. You end up with less damage, less healing, lower DCs and lower spell penetration (admittedly, spell penetration isn't very important for a Druid). Your pet is also less durable and effective.
Now, certainly, if you get buried under an army of enemies and somehow manage to not die after each of them hits you 5 times, you can do better damage with your otherwise not-terribly-impressive Light damage spells. But given that your Light damage effects are way down the list without such a bizarre setup, this isn't actually much of an edge - you're just buffing the terrible parts of your rotation, not the good ones.