Heroes 5.5 WarGame Edition - Tactical Edition.

Yes, the ballista is upgraded either through ballistics or artillery. These are two alternative upgrade paths. Artillery and Sulfur Rain, by the way, destroy 10-20% of enemy troops even before the battle (not just during sieges), so they are very effective perks.

Fire Arrows is too powerful a perk to include in the main ballista upgrade path. Currently, its purpose is to give a good ballista to races that have poor stats for it (due to the additional fire damage).