The only things they gain from Practiced Spellcaster are:
Full Level-based damage (which only help spells that don't max out at level 15)
Full Duration (which would be, in this case amount to extending a spell by 25%)
and
+4 to Spell Penetration
I don't think that's overpowered, especially since it doesn't give you what you're really missing from not taking those Sorcerer levels: the capstone, the level 18 PrEs, a 7th, two 8th, and three 9th level spell slots, and tons of mana.
I think I see your problem, you think the feat shouldn't provide a noticeable effect or make things more usable. What, do all your barbarian builds load up on Skill Focus: Perform?