Casting time: full round action, while grappling

[Infiniti2000]

Yes, that restriction is a pretty big one, especially for this reason. The wizard IMC always has a silent dim door ready. The sorcerer cannot (sans arcane prep feat) benefit from it for the same reason (i.e. escaping from a pin). The full round action is what we call double jeopardy (balance issues notwithstanding*). It makes the sorcerer pay twice for metamagic: +N level and increased casting time. Further making it difficult by allowing full round actions in a grapple but with a yet higher concentration check could be called triple jeopardy (again, balance notwithstanding).

* I say this as an attempt not to derail the thread. Metamagic + sorcerers gets a lot of people very vocal, KD and Thanee come to mind , so I'm just pointing out a potential issue so the OP is aware of it.