1st off, Jeff, U want to check your alt. output. I check at idle and no load, then small load, followed by 2000 RPM and heavy load. A load is anytime something turns on, ie. fan, lights, a/c etc.
At idle, no load, u should have 14.5-.75 volts. This takes into account 1. fuel pump 2. ignition system 3. ECU.
Idle, slight load such as fan(s) or lights U should have approx. 13.75-14. volts. A/C on, 13- 13.5 volts.
2000 RPM, heavy load, A/c on, lights on brite, wipers, anything U can turn on that will put a load on the alt. I want to see 13.75-14.25 volts. If not, look hard at the alt.
Now, as VOLTs go down, AMPs go up, thats a reflection of load. Thats what U want. U will have to check with FAST for there amp draw for the ECU, but I would guess it is very small, very, very small, compared to the other parts.
Pumps, fans, lights etc., these can all draw 20 amps or more, EACH. Start adding these up and you can have at a minimum, 60 amp draw at any time. This is why a very high output alt., even from the factory, are required for FI. Most factory alt. are in the 130 amp range.
What increases these draws above the stated factory rating? Wires that are to small for the load, poor connection due to either corrosion or loose fit. Bad grounds. Bad units. HEAT, and you can't beat heat, only manage it.
Wire from the alt to battery connection should be at least 8 gauge. From battery to relay to unit... 10 gauge. One relay per unit. 2 fans=2 relays. Use a high strand count wire, electricity travels around the wire strands, not thru them.
As for the ECU correcting for the voltage loss..... it always will. As the engine heats up, the resistance in the injectors and other sensors goes up, reducing voltage. To verify, measure the ohms across an injector when it is cold, then drive the vehicle ten minutes and remeasure the same injector. Then fig'r it for all 6 or 8 and you will see why the ECU will always have a voltage correction. It's the ECU's way to adjust to keep the injector on time correct.
All parts..... ignition boxes, ignition, fuel pumps- factory and aftermarket, everything has a rated amp draw. Start checking them, adding them up, etc. and U will see it better.