The main different by these upgrades are the armament, shield and hull strengths, troop capacity, and starfighter capacity. Although Telan already posted a summary of Star Destroyer classes, here are my thoughts on them:
Mk I Star Destroyer: The first of the batch. Armament was uniform at 60 each Heavy Turbolaser Batteries and Heavy Ion Cannons, with Starfighter Squadrons consisting of 1 TIE Interceptor Squadron, 3 TIE Fighter Squadrons, 1 TIE Recon Squadron, and 1 TIE Bomber Squadron. Troop Capacity is 9,700, with a number of AT-ATs, and AT-STs.
Mk II Star Destroyer: Improved version of the Mk I after some shortcomings in combat, as well as improved armament. 50 Heavy Turbolaser Batteries, 50 Heavy Turbolaser Cannons, 20 Ion Cannons, 2 TIE Interceptor Squadrons, 2 TIE Fighter Squadrons, 1 TIE Recon Squadron, 1 TIE Bomber Squadron. Troop complement the same.
Mk III Star Destroyer: A specialized Star Destroyer modified by adding Gravity Well Projectors to it. Also called a Dominator class Star Destroyer. Shields and Hull Armour reduced. Armament: 40 Heavy Turbolaser Batteries, 20 Heavy Turbolaser Cannons, 20 Ion Cannons, plus 4 Gravity Well Projectors. 4 TIE Fighter Squadrons. Troop Complement: Much reduced.
Mk IV Star Destroyer: Seen in the Hand of Thrawn Duology, in the form of the Chimera, and in her engagement against the Cavlirhu Pirates in the Pesitiin system. The Imperials take a cue from the Rebels. Also some technology is added, including a combat predictor, and a cloaking shield. Armament: 50 Heavy Turbolaser Batteries, 25 Heavy Ion Batteries, 60 Heavy Proton Torpedo Launchers, arrayed into four clusters of fifteen each. Armour is the same, Shields lowered a little bit. Starfighter compliment different to reflect decline in Imperial starfighter capability: 3 Squadrons PreyBirds, 2 Squadrons TIE Fighters, 1 Squadron TIE Bombers. Troop Capacity: Probably a little less.
Mk V Star Destroyer: Introduced in later NJO, which doesn't affect our timeline. Ground Troops completley removed, and the ship becomes a purely space battle and ground bombardement killing machine. From what I've seen, armament is as follows: 40 Heavy Turbolaser Turrets, 40 Heavy Quad Laser Turrets, 20 Ion Batteries, 60 Proton Torpedo Launchers. Starfighter Capacity is also increased with the elimination of most troops, as seen in the battle of Escambia. Likely Starfighter Squadrons as follows: 2 TIE Defender Squadrons, 4 TIE Interceptor Squadrons, 4 TIE Fighter Squadrons, 2 TIE Bomber Squadrons. Troops: Maybe a battalion or two, but not more.
Mk I Star Destroyer: The first of the batch. Armament was uniform at 60 each Heavy Turbolaser Batteries and Heavy Ion Cannons, with Starfighter Squadrons consisting of 1 TIE Interceptor Squadron, 3 TIE Fighter Squadrons, 1 TIE Recon Squadron, and 1 TIE Bomber Squadron. Troop Capacity is 9,700, with a number of AT-ATs, and AT-STs.
Mk II Star Destroyer: Improved version of the Mk I after some shortcomings in combat, as well as improved armament. 50 Heavy Turbolaser Batteries, 50 Heavy Turbolaser Cannons, 20 Ion Cannons, 2 TIE Interceptor Squadrons, 2 TIE Fighter Squadrons, 1 TIE Recon Squadron, 1 TIE Bomber Squadron. Troop complement the same.
Mk III Star Destroyer: A specialized Star Destroyer modified by adding Gravity Well Projectors to it. Also called a Dominator class Star Destroyer. Shields and Hull Armour reduced. Armament: 40 Heavy Turbolaser Batteries, 20 Heavy Turbolaser Cannons, 20 Ion Cannons, plus 4 Gravity Well Projectors. 4 TIE Fighter Squadrons. Troop Complement: Much reduced.
Mk IV Star Destroyer: Seen in the Hand of Thrawn Duology, in the form of the Chimera, and in her engagement against the Cavlirhu Pirates in the Pesitiin system. The Imperials take a cue from the Rebels. Also some technology is added, including a combat predictor, and a cloaking shield. Armament: 50 Heavy Turbolaser Batteries, 25 Heavy Ion Batteries, 60 Heavy Proton Torpedo Launchers, arrayed into four clusters of fifteen each. Armour is the same, Shields lowered a little bit. Starfighter compliment different to reflect decline in Imperial starfighter capability: 3 Squadrons PreyBirds, 2 Squadrons TIE Fighters, 1 Squadron TIE Bombers. Troop Capacity: Probably a little less.
Mk V Star Destroyer: Introduced in later NJO, which doesn't affect our timeline. Ground Troops completley removed, and the ship becomes a purely space battle and ground bombardement killing machine. From what I've seen, armament is as follows: 40 Heavy Turbolaser Turrets, 40 Heavy Quad Laser Turrets, 20 Ion Batteries, 60 Proton Torpedo Launchers. Starfighter Capacity is also increased with the elimination of most troops, as seen in the battle of Escambia. Likely Starfighter Squadrons as follows: 2 TIE Defender Squadrons, 4 TIE Interceptor Squadrons, 4 TIE Fighter Squadrons, 2 TIE Bomber Squadrons. Troops: Maybe a battalion or two, but not more.
