The argument that later possible entry in a prestige class should allow greater benefits is not a very good one, even if it is presented in some core book.
It reinstates the Wizards role from the former editions, weaker than everyone at lower levels, but stronger than all others at high levels. That might be cool in a novel, in a game its just annoying. First for the player of the wizard, later for everyone else.
And that even only applies if the character is played from low levels and up. But the same prestige classes is as likely (or actually more likely) to be used when replacing high level dead pc's. With one fell swoop you have avoided the dreary low levels (or even medium levels for the suggested prestige class) and gone straight for the power levels where you leave everyone else in the dust.
The goal to strive for is that all classes should be equally powerful at all levels. Thats obviously not the case or perhaps even possible. But to allow such a class is definately a step in the wrong direction.
Another thing to consider: while this may look overpowered compared to an arcane-based prestige class, take a look at the divine-based prestige classes: d8s, cleric BAB, two good saves, special ability and spellcasting every level.
Comparing this to a cleric-based prestige class, you drop a hit die level, a good save, and all the per-level special abilities in exchange for sorcerer spells.
Thats still a very good deal in my opinion. The clerics fighting power at high levels clearly comes from buffing spells. Without those he will be crushed in melee by appropiate foes. So taking away a few hp won't change that at all, with divine power, divine favor and righteous might running he's a powerhouse anyway.
That gets even worse when he can also cast spells as shield, Improved invisibility and polymorph. It also fills out the weakest point of a cleric type, ranged combat, with lots of nice area of effect and ranged touch attack spells.
I'm sorry but it is just a munchkins dream.
).
