On my '87 when I remove the oil cap while the engine is running (cold or warm) I notice no difference in idle at all. Like none. I don't have the stock air box on the car, however. Rather, it's one of the cone type filters. I don't know if this is the "problem" with having no dropout or not. I've replaced CHT, speed and reference sensors, O2 sensor, and the large hose from the tank. None of these made a difference. My tech removed my intake manifold last year to replace fuel lines, and those gaskets were replaced so I know there is no leak there, either.
My car runs fine and has no hesitation. The only time I EVER had any hesitation was when the idle control valve went south. I cleaned it a few times and it helped very temporarily. Idle was rough, and I'd notice major drop outs under acceleration. Replacing the ICV fixed this. However, my idle was rough and you said yours isn't.
The jumper test they're talking about is outlined here (which is taken from the Steve Wong instructions after changing to his chip):
I remember reading that you can test your brake booster by pumping up the brake pedal with engine turned off. Put your foot on the brake and start the car. If the pedal sinks, then the booster is properly pulling vacuum to pull the pedal down. If not, the booster is suspect or its vacuum hose.