If this is the case with your engine then I am sorry to inform you that your engine is of poor design and or poor assembly. I am afraid that your statement above is incorrect with respect to what is possible.What difference does it make if the valve events are perfectly balanced for each direction of the stroke? because the cylinder/piston arrangement isn't balanced in both directions, nor can it be. The bottom piston face has a void in it's surface area where the piston rod connects. This causes an imbalance in displacement between the upper and lower cylinder and it causes steam engines to lope and there is virtually no way to remedy it. Practically or by worthwhile means anyways.
The piston's position on it's rod can/should be adjusted to set the un-swept volume to match the swept volume at both ends of the cylinder so as to achieve a balanced compression ratio, this volume includes the volumes of the ports all of the way to the valve faces, and takes into account the volume that is displaced by the piston rod. If there is not sufficient adjustment available to achieve this, then the piston shape is incorrectly designed. It is quite easy for a fitter to measure and adjust the volumes and it was indeed common practice to do so. If the compression at each end of the cylinder is not put into balance, it is impossible ( probably should say "rather difficult") to set the engine's compression via the exhaust lap on the valve/s to achieve correct compression at the end of the stroke. If the compression is too high or too low, the performance of the engine, and it's life, can be greatly hampered. The volumes that I mention were simply measured using water, no computer or fancy simulation necessary. In larger marine engines, especially engines with insufficient counterbalance or no counterbalances at all on the crankshaft, the compression was made higher at the bottom than the top, and the lower inlet was opened slightly earlier, both to help balance the engine, but this is not really necessary in a launch size engines.
As i mentioned earlier, in loco practice it was common to work toward less than 1% error in valve events, this was done and can still be done without a computer. It was probably also common practice with the more professional marine engine builders/fitters to work toward similar tolerances, it is not much fun to run out of fuel in the middle of the Atlantic.
Steam engines are inefficient, some steam engines of seemingly identical design are a lot less efficient than others, but they don't necessarily need to be. A poorly set up steam engine can easily use twice the steam for the same output (or half the output for the same amount of steam) as the same design engine if it were set up correctly.
So yes, you are correct that if the boiler cannot keep up with the engine then the engine's ultimate power is limited. The part that you have left out is this, if the engine is wasting steam because it is incorrectly set up, this may be in fact be the cause of the boiler not keeping up.
While I agree that a hobby launch owner may not have sufficient interest or motivation to go chasing better efficiency or more power, it can be nice to only need to carry half the fuel for the same steaming distance and lighter boat is easier for the steam plant to push.
cheers
Lionel