I don't think GC needs a complicated revamp to fix it. Simply,
Change it to give a flat recovery AP and max AP bonus of 4
It makes Constitution 50% less effective
This lowers their hp, gives them a sizable bonus to AP, encourages the player to keep dumping points into speed, and discourages them from pumping their constitution back up.
I think you're right that it doesn't need a complicated revamp. I think your revamp isn't quite the obvious direction to take it though.
Here's a real simple fix:
Glass Cannon:
+ X% damage dealt
+ Y% damage taken
X and Y could tuned to whatever seems reasonable. Point is removing AP from the equation. Almost any large AP boost is always going to be a no brainer talent unless it has ridiculous downsides.
Since extra AP is much more than just offensive power, it never made sense for the talent. It allows a player to use extra AP to do more both offensively and defensively.
