What is it that WotC did that created this mechanic?
In Complete Arcane, a followup to Dragon Heritage is Dragon Breath, which converts spell slots to a linear area effect attack that does 2d6 per spell level used to power it. PHB2 does the same with Infernal Howl, though the area is conical.
All SPR does is take this to the logical extreme, namely converting all of a PC's arcane spell slots.
No, SPR doesn't just look at slots.
Yes, it does. See WotC's mechanics spelled out above.
Using your SPR metric to compare these classes/PrC yields a lop-sided result. Precisely because 9 1st level spells are not equal even one very weak 9th level spell. Yet SPR says they are equal. That is precisely what SPR is measuring.
Again, I conceded that point (several times), as well as stated that SPR isn't meant to be a single metric AND countered that spell quality within a spell level can't be measured by a statistical comparison (see previous posts).
The utility of SPR is that it provides a statistical metric for comparing arcane spellcasting classes without dealing with variables that don't have common denominators.
Your metric allows the Theurge-type to have a large number, much larger than the single-classed wizard, precisely because it chooses to cheapen the value of the high level spell slot, and in doing so, it places a greater premium on low level spell slots.
You conceed that the ability you are using to create this metric in and of itself, denigrates the power of the high level spell. In other words this metric is flawed, as a method to compare relative power.
You are taking a single feat, and making it to be more than it was ever designed to be. It is flawed, precisely because it places a higher premium on the low level spell slot, and places a lower premium on the high level spellslots.
Yes. Your points (and others) are why this isn't intended as a sole metric of comparison.
(emphasis mine)If the actual average increase in power level between levels is 2,5 (just speculating)
Therin lies the problem from the other side. How does one calculate average increase in power levels? As I've pointed out, the very nature of spell design prevents us from comparing power levels within a spell level, much less across them.
As another comparison, we may look at Innate Spell
Yes, we can.
How does that unlimited number of a 1st level spell compare to SPR?
While interesting, its quality varies specifically with the spell chosen to be innate- the problem of spell quality that I've been talking about throughout this thread.
If the Innate spell had been chosen by an Enchanter with Evocation as a barred school, he'd probably choose Sleep or Hypnotism as the spell in question.
What if we look at a a Conjurer who took Summon Animal 1, a Diviner who took True Strike, an Illusionist who took Color Spray, a Necromancer who took Ray of Enfeeblement or Fear, or even a Transmuter who took Enlarge Person or Magic Weapon?
Those don't really translate into comparable outputs with the Innate Spell analysis. One gives you a bunch of critters which may or may not get to do damage, a couple of them give you a dependable bonus to attack, another couple remove opponents from a fray without killing them. Some buff, some debuff.
IOW, zero mathematical comparison is possible.
With SPR, its all boiled down to damage in a day.