If you were to design 2 engines (one reusable, one expendable) with the same Thrust, Specific Impulse and Chamber Pressure:
the reusable engine would be heavier, more expensive, or both. For a reusable engine, parts have to be designed with a much longer lifetime than for an expendable engine. You can do that in two ways: make the part heavier to provide more wear margin, or use better materials that wear out more slowly.
The reusable engine would probably be more complex too: you'd want to design it to be easily accessible for inspection and (if necessary) part replacement. So you'd have access panels, piping with bolted flanges instead of welds etc. Maybe you'd move the turbopump to a place that's easily accessible (which needs more piping than the usual place on top of the combustion chamber).
When you want to increase performance of an existing engine, you have only a few options. You can make combustion more efficient (unlikely, because you already have efficient combustion to start with). Or you can inject more fuel and oxidiser. When you do that, combustion chamber pressure increases. Pressure in the turbopumps and piping increases as well.
These components are built to withstand a certain pressure. This design limit is a bit higher than the normal operating pressure. This is the margin you can work with.
Materials have a certain tensile strength, this depends on their temperature. In a rocket engine, components have to be kept below the temperature at which a metal would start to soften. Above this temperature, the component will break down rapidly.
If you take a reusable engine, and use it in expendable mode, the only way you can improve performance is by using this margin. If the engine has to run for 2 minutes, you can try and run the engine so that the components reach their critical temperature at 2 minutes 15 seconds, when the stage has burned out and the second stage is far enough away that a first stage explosion won't damage the second stage.
This is a difficult proposition though. When a material approaches its limits, its behavior is not linear. Say you've got an engine that can run indefinitely at 1000 ºC. If you raise its operating temperature by 5%, engine life will not drop by 5%. It might still be good for indefinite operation at the new temperature, or it might melt after 30 seconds. This depends on the detailed design of the engine and the margins that were used in its design.
The critical parts aren't wear parts: a combustion chamber is not designed to lose material as it is used (ablative lining, for example). If this were the case, it'd be easy: just run the engine to burn off the ablative lining in one go instead of making it last 10 missions.
How much performance you can gain this way depends on the exact engine design: the materials that were used, the margins used by the designers. My guess is, in a mature engine design you'll have very little margin to work with. Too much margin means the engine is too heavy.
Here's a paper that gives some insight into the design decisions that were made to make the SSME reusable. Reading the paper, I get the impression that the main differences were in the design process itself: they had to do research to find materials and component detail design that would be durable. So the design process was more expensive because they'd need to advance the state of the art, finding more reliable materials and construction methods.
They also needed to design the engine for maintainability. They needed to make sure wear parts were easily accessible to minimize the time spent on maintenance, they had to provide inspection panels, create tooling etc.
The Orbiter main engine is the first large liquid rocket engine designed specifically for a long service life. The engine is capable of completing 100 starts or 7.5 hours of operation between overhauls. ... Specific changes to earlier engine designs were necessary
to fulfill this long service life requirement. However, in each case, the weight of the design change was evaluated for its impact on engine weight-to-orbiter payload capability. Some of the engine systems and components designed for long service life are the hot-gas system, turbomachinery, and valve seats.
All turbopump seals operate with a positive clearance to prevent wear and ensure long life. Low bearing loads are ensured by a balance piston system within the turbopump that reduces axial shaft loads. Turbopump bearing life is determined by rolling-contact fatigue, which is a function of speed and load. ... The use of vacuum melted materials further increases life so the average predicted bearing life is approximately 65 times the B, life value.
A retractable seal for the propellant ball valves is a feature unique to the engine, added specifically to provide long life and reusability.
Automatic checkout, operational monitoring of flange leakages, and automatic propel!ant valve seat leakage detection have replaced the manual leak and functional techniques used on previous engine systems. Life monitoring techniques of internal inspection, maintenance instrumentation, and drain system leak checks resulted from a maintain ability analysis conducted early in the design phase. Since corrective maintenance represents the largest single expenditure of resources during the SSME turnaround cycle, hardware accessibility and handling were emphasized early in the design phase. This maintenance concept results in a maintenance cycle for the three Orbiter main engines requiring an average of 25 hours of the 160-hour Orbiter turnaround.
a major effort was expended to provide full internal inspection capability. This effort defined requirements, selected equipment, scheduled usage, and designed access ports.
The SSME was tested to at least 111% of rated power. For flight, 109% was the maximum.
This paper summarizes the changes made to the SSME over the life of the program. In several increments, they reduced the number of components and the time required to make them.
Another thing to look into is the RS-25E, the expendable variant of the SSME that is being planned for the SLS. Although as far as I know, they aren't planning any performance increases for that, just a decrease in manufacturing cost.