Effects that affect a creature’s power (such as the one from Glorious Anthem, for example) apply only to creatures on the battlefield, not to creature spells on the stack.

If a creature card has “*” in its power, the ability that defines “*” works in all zones, including the stack.

Any spells or abilities that would counter Spellbreaker Behemoth or (once it’s on the battlefield) a creature spell you control with power 5 or greater can still be cast or activated and will still resolve. They just won’t counter that spell. Any other effects they have will happen as normal.