Unfortunately if you increases the bosses cast time it trivialises some fight mechanics.

For example, PLD stun is on the GCD, so if he isn't careful about the timing of his attacks, he wont be able to stun that deadly move! The WAR has the advantage that his Stun is off GCD, so he has an easier time preventing that deadly move, but of course the job has its own drawbacks to balance this.

If you extend the cast times of boss spells, the WAR loses its advantage as any old pld can just wait until his GCD is ready and still perform the stun.

The Current spell cast times etc are perfect just the way they are, they provide a reward for people who time their attacks well without being to loose to make it so anyone can do it.

As a side note, the stun effect normally occurs at the end of the animation, so you cant just hit your shield bash button when the spell is at 90%, that simply wont work.