Enhanced Firepower strengthens the volley your Rooks fire when you send them into battle. It is a Ringleader upgrade needed before the top training tier.
Enhanced Firepower is entry 4 of 17 in the Ringleader branch, one of the three branches (Ringleader, Insider and Swindler) on the Rooks' 39-upgrade board. It is bought with money and materials, not skill points: £6,000, 100 Metal, 75 Chemicals. Before Enhanced Firepower unlocks you need Training Rooks II.