I'm quite astonished.. ..well, one thing is debating the balance of Spellcasting prodigy compared to the D&D general balance, another is comparing SP with Weapon Focus.. ..ya, wow, a +1 to hit, and at mid to high levels, you'll possibly hit once more, while the mage is incinerating armies.. ..I mean, it's like comparing apples to oranges, a +1 to hit means nearly always a d8/10 +15/20 at high levels, while more spells and +1 DC means a monster is dead.. ..or, want to talk 'bout the benefits for a Wizzy/Cleric? Oh, yeah, double benefit for a half effective character, since all he can cast @ CLvl 20 is 5th level spells, cm'on..
Seriously, the balance issue is campaign and -more- players dependant.. ..if you play spellcasters easy, it's not that much a problem.. ..if your players use spellcasters as artillery, or save-or-die machines (and this is possible @mid/high lvls), the SP feat is just another issue just like the Madness Domain, the "evil" PrCs in the FRCS, the Archmage (not the Hierophant), et similia... ...there must be a balance between spellcasters and the world, that means also between magic user players and monsters, and magic user enemies and players... ..it's not fun at all to see your Red Wizard cast an Eyebite with DC 30 at level 11, nor its fun to make your players save against a similar DC at a similar level.. ..they clearly cannot do it, nor do the monsters..
AO, you speak of a Sorcy with CHA 34 (+12), SP (+1), Spell Power +3.. ..ok, so your DC are 26+Spell Level.. ..considering a Sorcerer has at his disposal spells that have a save against all three save types, are you really telling me that you are quite often resisted with a Fort DC of 34 (Horrid Wilting), or Ref DC 33 (DBFireball), or WIll DC 32 (Eyebite)? or 36/35/34 if you use Spell Enhancer before? IME, if you choose carefully the save type and you have that DC, the spell 80% of the time is not going to fail.. ..if that is not the case with you, then you DM has probably adjusted up a bit the save bonuses, that means you are freely allowed to pump up your DC without unbalancing the campaign.. ..all in all, it's always a matter of spellcasters/world balance, as I said...
P.S. [My curiosity] 34 Cha @ CLvl 17 w/out Eagle's Splendor? Mmm, 34 -5 (tome) -6 (Cloak of Cha) - 4 (Level ups) = 19.. ..are you an Aasimar?