The FSX-- which in real life was never produced-- is one of the most powerful planes in the game. Even without any power-ups, its Ninja Missile is strong; on full power, it is devastating to the enemy. The Ninja Beam fires a wide beam of energy from the plane capable of immediately destroying smaller enemies.
FS-X Special Version
"The ferocious fighter has been rebuilt by a famous Katana-craft master. Loaded with guided missiles."
-Aerofighters 2 English game manual
The FSX retains its excellent weapons from previous games; when powered up, in addition to its normal artillery, it launches guided missiles that automatically attack the enemy. It once more features the powerful Ninja Beam as its bomb.
"The FS-X is perfect for top gun pilots. It banks quickly and performs fast barrel rolls, keeping the best pilots on their toes. Because of its squirrelly attributes, it's a hard jet to control, but a good choice for dog fighting."
-from the Aerofighters Assault English game manual
In Assault, the FSX is again an extremely powerful plane, especially its Stars missile. It's especially useful when the player needs to eliminate a lot of smaller enemies quickly.