I've pulled and plugged what I believe to be every vacuum line in the engine bay, for no lead on the problem

It still may be a ruptured EGR diaphragm which is letting exhaust bypass into the intake even with the vacuum source removed. Check the temperature of the pipe connecting the EGR valve to the intake manifold as Marlin described. If the EGR is properly closing and restricting hot exhaust gasses from passing, then the connecting pipe will more or less be the same temperature as the intake manifold.

Reading your opening post, my initial guess was an electrical issue, and I was going to mention either a bad igniter or a bad ground at the igniter (ie. as the RPMs increase so does the load/amperage through the electrical system, whereby a bad ground would handle low load levels but not high load levels). This happened on my MR2 when I had a normally aspirated engine in it, somewhere around 6,000 rpm and above the power band dropped off and it was running like crap. I added a few grounds in troubleshooting and it not only fixed the power loss but I also had a smoother idle. Ad a few misc temporary grounds: one to the block, one to the head, one to the alternator housing, and one to the body. It couldn't hurt and would be a quick way to trouble shoot a bad ground.

Good luck
BigMike