This does make evocation blasting more useful. Unfortunately, it also makes conjuration blasting more useful too, at exactly the same rate, which is not exactly ideal. (Uncapped orb of acid with +2 CL and 50% damage against acid-immune targets? Do want!) It also doesn't require arcane casting specifically, which does weird things when you slap it on a Druid or Cleric or Artificer. There are a few non-blasting spells that have energy descriptors as well, which makes things weird and nasty. (For example, [fire] does double duty as an energy descriptor and a planar origin descriptor, and uncapping lesser planar ally and lesser planar binding, even for [fire] creatures only, is not a good idea.)
I would peg the lost spellcasting at first and possibly third level (and you should probably put another entry for Elemental Affinity on the table at that spot anyway, like "Elemental Affinity (Chosen Element) +1" and "Elemental Affinity (Chosen Element) +2"). Also, cut the uncapping down to a far more sane version, such as raising the cap by your class level (+1 to +5, in other words), or even less. So far as I know, Reserves of Strength is the only way to fully uncap spells in existence, and that's one uncapping method too many.