One thing that jumps out at me immediately is that if you restrict full-round actions, you harm martial characters far more than casters. I would instead set a limit of 1 non-quickened spell per round, regardless of how many standard actions you have, so that casters do not get overly OP.
I get what you're saying, but my goal for this was to be an alternative to a full-round action, not a straight out boost, that's why I'm wary of letting it give full-round actions. Although I'm mostly thinking in terms of "full attack" vs "5 standard attacks", so maybe that's what skews my opinion.
It makes gishes supremely powerful as currently construed - they can toss off 4 spells a round instead of a normal caster's 2. Likewise, it massively boosts ToB classes - they can now chuck in 4 strikes a round instead of just one. As a crusader, that means a never ending spam, for instance.
That's not necessarily a bad thing. Or a good thing, for that matter.