Lowering the fuel pressure for idle quality could result in fuel starvation up top. Yes, maybe for your naturally aspirated combination 24s would be fine. But you mentioned the NOS shot. You would need at least the 30s at that point.
A programmable computer would give you the flexibility to tune for idle and WOT, and allow you to grow with combination changes. I run an EPEC and I know its alot of coin, but you can adjust with combo changes, smooth the idle and pass emissions inspection.
You hit the nail on the head though. Your dyno session will tell the story. Never mind what I say, the truth will come out on the dyno (with wide-band O2 sensors). Here's what you'll discover:
You can't lower the fuel pressure enough to obtain decent idle quality. Why? Because its not the injectors alone - its the cam.
When you lower the fuel pressure, you'll find your getting a bit lean up top. When you tell the mechanic you'll be adding a 150 hp nitrous shot to this, he'll just laugh.
Good Luck!