Well, as far as the Saladin having the firepower of the Constitution, that was the idea. In SFB (Star Fleet Battles), the Saladin-class destroyers do have the same firepower as the Constitution, but not the power to run all systems at full capacity. In starship design terms, it was underpowered. But it was manueverable. And you also don't have to spend every bit of space allotted to a size category.
Saladin-class Destroyer
PRODUCTION DATA
Origin: UFP
Class and Type: Saladin-Class Destroyer
Year Commissioned: 2255
HULL DATA
Structure: 30
Size/Decks: 5/10
Length/Height/Beam: 242.5/60/127.1
Complement: 200
OPERATIONAL DATA
Atmosphere Capable: No
Cargo Units: 50
Life Support: Class 3 (D)
Operations Systems: Class 3 (D)
Sensor Systems: Class 3 (+3/D)
Seperation System: Yes (Emergency)
Shuttlebay: 1 aft ventral
Shuttlecraft: 5 Size worth
Tractor Beams: 1 fv
Transporters: 2 standard, 2 cargo, 2 emergency
PROPULSION DATA
Impulse System: SBE (.6c) (D)
Warp System: PB-32 Mod 3 (6/7/8) (D)
TACTICAL DATA
Phasers: Type IV ( x3,B)
Penetration: 4/3/3/0/0
Torpedo Launchers: Mk 12 DF (x2,B)
Photon Penetration: 3/3/3/3/3
Deflector Shield: PFF 2a (B)
Protection/Threshold: 13/2
MISCELLANEOUS DATA
Manuever Modifiers: +1C, +3H, +3T
Traits: Nimble, Enhanced System (Impulse, +.1c)
Notes: Throughout the 23rd century, there was a chronic shortage of starships. The Saladin-class project was a measure to increase the number of ships in the fleet and not break the Federation's budget. While the Constitution-class heavy cruisers were the state-of-the-art, they were also incredibly expensive. The Saladin-class was designed to give the same performance at half the price. The program was not a success initially. The first run of Saladin-class destroyers were notoriously underpowered. They underwent a refit in the 2260's and have been a common sight throughout Federation space since. Being as fast as the Constitution-class cruisers at warp and faster at sublight, they got to hotspots along the Klingon border and Romulan Neutral Zone quickly. They were also more manueverable than most ships their size. Their firepower was the equal of the heavy cruisers but their shields were not quite as strong. The Saladin-class was not expected to go toe-to-toe with a D-7 and win, but it was expected to buy time and to give the Klingons a bloody nose. This was also one of the few starship designs to utilize only one nacelle. A specially designed set of warp coils went down each side of the modified nacelle to form the warp field around the ship. While they were well equipped, they did not perform as well as the Constitution-class in the long term exploration role, but did perform well in border patrol and picket duty missions. They also did well as scouts, so well in fact, that a dedicated scout version was produced in limited numbers. It removed the torpedo launchers and upgraded and enhanced the sensor systems. This Hermes-class scout become the premier scout and science and research vessel in starfleet, until the commissioning of the Miranda-class light cruisers.
"Retreat?! Hell, we just got here!", annonymous American Marine, WWI
"Gravity is a harsh mistress....", The Tick