Hello Querty and welcome to JA. Just so you don't think this site is completely bad, I am going to jump here and work with you on this.
You seem to know much about what's going on with your engine, but I do have to clarify a few things. First of all, the cylinder performance test can not verify a dead injector since it only measures the time it takes for the piston to make its travel, there is no such sensor within the cylinders to measure the amount of explosion or force, or even temperature of the burn. Cylinder 1 is 16% slower the rest, but this does not necessarily mean the injector is dead. In a diesel engine there are more factors involved in the power stroke. One of them, and most important is air/fuel ratio. This is accomplished by the amount of air and temperature of the air that is let into the cylinder vs. the amount of fuel injected into the cylinder. If you indeed have an intake valve that is not adjusted properly this can affect the air/fuel ratio thus causing a cylinder to be stronger or weaker than the others. This is why step 5C-1 of the Cummins guided diagnostic tree requires that you check you overhead adjustment. The only absolute way to isolate a bad injector is to perform a cylinder cut-out test. An ISX engine will run on 1 cylinder at a time and still maintain the RPM. If you cut-out 2-6 and the engine is still running at 700 RMP (or whatever speed is preset for the test) than this would mean that 1 is still firing.
But, to throw another wrench into it, an injector may be working but not at full capacity. Injectors will wear unevenly due to the fact that they consist of moving parts at close tolerances. Change this tolerance only slightly and you can have an injector that does not perform as the rest. Heat and also the machining of the individual parts can have a factor in their life. In my many years as a mechanic I have seen many times when only 1 injector will fail at a time. In regards XXXXX XXXXX adjustment of an injector, there are many factors that can change this. The spring becomes weak, the roller wears slightly, the cam wears slightly. I have seen cams where only one lobe is worn and all the rest are good. This can only be found by checking the adjustment of the injector.
In regards XXXXX XXXXX fuel being trapped, this is not true, the fuel in the rail is maintaining a pressure of 230 - 274 psi(w/EGR) and 225-274 (wo/EGR) at an idle, and the amount the injectors use will not affect this. Fuel is circulated through the rail in a continuous motion and is controlled by the Pressure Regulator.
Back to the original post, I assume you have a CM870 and not a CM875? Although they do have a CM871. Can you give me the S/N and the current ECM calibration code? There are several factors that can cause low power, but since you say it has had a quick drop, over a short period, we can eliminate a few of them.
The ones that jump out first would be a leak in your charge air circuit, turbo going out, and a clogged air or fuel filters. I assume you have checked these, correct? What is the measure fuel pressure? Not the pressure that is indicated in the ECM as a bad sensor can give a false reading, but the actual fuel pressure measured by a mechanical gauge. Now if all these check out, we can get into checking other more technical problems that could cause low power.
Thanks for coming to JA and I hope we can work together to solve your problem.