Beam Overload applies to the next beam attack to fire (within 30 seconds), so yes, firing your most powerful beam will likely yield the best results.
Cannon Rapid Fire however applies to all cannon/turret attacks for the next 10 seconds after activating the ability. Normally, your best plan here is to fire everything.
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Cannon Rapid Fire however applies to all cannon/turret attacks for the next 10 seconds after activating the ability. Normally, your best plan here is to fire everything.
CRF enhances all cannons for 10 seconds.