I'm not saying it is how it is supposed to work. But, I do not believe fuel pressure is an input to the computer, so, it is blind there. It has a nozzle (the injector) and it "knows" the size. But, you can lie to it. Tell it the injector is 19#/hr and install one that is 24#/hr. With 50 psi fuel pressure, it will leave the injector open some period of time with the 19# injector, seeking the correct reading on the Oxygen sensor. With a nozzle about 25% larger (24# vs 19#), and pressure about 25% less (40 psi vs 50 psi), I believe the time the injector pulses for a 24# injector and 40 psi will be about the same. At least I expect it will run. Then again, the computer can adjust some based on the Oxygen sensor reading.
Seriously, if that won't work (and I'm not saying it is the right thing to do) I'd love to understand why.
I guess maybe I got confused by your question when you said "will the computer think 19# injectors at 50 psi are the same as 24# injectors at 40 psi". There is also no input in the computer for injector lb #. The computer uses things like voltage offset, high slope and low slope, breakpoint, to set the fuel trims. Different injectors have different ratings at different PSI. When you increase the PSI it just makes the injector act differently than the computer is expecting, it doesn't just think you dropped a bigger injector in there, you're just forcing a rich condition.
I couldn't find 19lb injectors at 50PSI but below is an example of ratings for a 24lb injector at 55PSI vs 30lb injector at 39 PSI. Maybe the difference isn't noticeable to us when driving but the computer knows.