Do two engines with the same specific impulse always perform the same amount of work? As the title say, do engines with different thrusts with the same $I_{sp}$ and the same amount of fuel perform the same amount of work?
 A: The answer is no.
A silly counterexample: Consider two rocket engines, one of which delivers 760,000 pounds of thrust at launch, the other, 100,000 pounds. Both have the same specific impulse. The rocket, engine structure, upper stages, and payload have a mass of 100,000 pounds, and the fuel tanks are filled with 500,000 pounds of fuel. (Sorry for the English units. This one of the areas where English units are much easier than metric, unless you use the forbidden kilopond. A lot of European rocket scientists still use the forbidden kilopond.)
The first rocket takes off. The engine does a lot of work, raising the rocket's altitude and increasing it's velocity. The second rocket just sits on the launch pad and never takes off. It does no work. This is a silly example because no one would ever attempt to use that second engine. That math is easy. Thrust needs to exceed total weight or the vehicle won't take off.
Here's a more realistic counterexample: Consider two rockets in a circular orbit. Both rockets have the same mass, the same amount of fuel, and have engines with the same specific impulse but different thrust levels. Both rockets use all the fuel in the most optimal fashion possible to reach the highest possible circular orbit. The rocket that reaches the higher orbit is the winner, and once again it's the rocket with the higher thrust engine that wins.
Low thrust comes at a cost. This is one of the factors that motivated the development of the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) engine. That engine has the ability to vary between low thrust with a high specific impulse and not so low thrust with a not so high specific impulse.
