I have a '99 stock xlt supercab rwd with the 4.2l v-6. I believe the engine is rated for about 200hp. The stock injectors are 24 lb/hr, which rates them at around a 70% duty cycle. My needs are modest for this truck, I'd like to change to perhaps 19 lb/hr in hopes of a little better flow control equaling a little better economy.
devils advocate: drop to 17 lbs/hr, would the results be limited hp? or ?
Thoughts?

 

Injection does not work like your thinking.
Injection works on a pulse width basis for the amount of fuel injected.
The 'width modulation' or open time is calculated by the computer before every intake stroke for every cylinder based on the Ox Sensor reading, throttle opening and RPM .
Going to smaller rated injectors will just lengthen the pulse width to obtain the same amount of fuel.
Only when the fuel becomes limited with the pulse width fully open will the power begin to drop and cylinders begin to go lean depending on the RPM, and Ox sensor readings or the fixed fuel tables under wide open throttle..
Mainly, you would see the motor begin to fall flat on power at some high RPM or call for more power. It would show up as a lean misfire condition
Injectors are normally flow rated at a nominal 38 lb pressure..
The sizing is kept small enough to meter fuel accurately at idle using about 5% pulse widths and not run out of fuel at the highest rated RPM and power the motor will make at about 80-85% to leave a little head room to be sure the motor does not run lean at max power.
If you check an injector table, it will show you the projected amount of power one injector will support times the number of cylinders.
Normally 19 lbs injectors can support nearly 300 hp on a 5L v8 under perfect control. That is 300/8 = about 37.5 hp per cylinder.
On a V6, 6 x 37.5 = 225 hp which is about right for this motor.
24 lb injectors are a bit on the large side for a V6 but the system could still handle them by adjusting pulse width downward.
Down sizing injectors does not equate to higher mileage, only the max amount of power they will support.
Engine displacement, RPM and expected power determine injector flow rating, and is independent of fuel consumption.
Modifying an engine for more power at higher RPM would be a consideration for larger injectors as well as the fuel system flow capability to support it. Under these conditions you could have large enough injectors but run out of fuel flow from the fuel pump so there is more to it than meets the eye.
Good luck.