So, after a bit of logging, I've finally narrowed down the fueling problem I've been having: my fuel pump. The stock Evo VIII fuel pump comes equipped with a pressure relief valve, which starts dumping fuel right around 65psi; with a stock base fuel pressure of about 43psi, that means you start dumping fuel at right around 22psi, and anything above that is pretty much a lean condition waiting to happen. So, your injector duty cycles spike suddenly, and you start knocking. The fix? A replacement 255lph high-pressure Walbro fuel pump, which turns out to be one of the simplest drop-in replacement items I've ever had the pleasure of installing (at least, as long as you install it when your tank is just about empty). Because of the unique design of the Evo's fuel pump relay circuit, you don't need to upgrade your fuel pressure regulator as part of this exercise, a welcome change from the DSM world. Injector duty cycles now get up as high as 93%, but that's much better than being completely out of fuel. I'll need new injectors before I can turn the boost up much more than a few psi more, though.
Another change I've made is the addition of a JDM MAP sensor (an original Mitsubishi part from the Japanese version of the Evo IX). This rather unremarkable direct replacement for the USDM MAP sensor gives you one very useful thing: the ability to both log boost (up to about 32 psi), and to alter the ECU to target a desired boost level, rather than a desired load. That makes for much more predictable tuning; rather than having boost spike during hot weather (which is exactly the opposite of what you want), it gives a repeatable boost target based on absolute pressure. One oddity to the ECU changes is that, because it's based on absolute pressure (rather than gauge pressure), you have to tell the ECU what your altitude is to get your calculations to work correctly. Where this becomes problematic is when you make a drastic elevation change (say, by going to a track in another state), but don't change the offset: your boost is suddenly lower or higher than before. Right now, I'm running about 21psi (only slightly higher than stock), and I'll probably bump that up a few psi before I'm done mucking with it.
Yet another change was finally getting the AMS downpipe that I picked up a few months ago onto the car. I bought a couple of 3" flanges and gaskets from Performance Autowerks, and using a bit of spare 3" stainless tuning I had lying around, welded up a rather ugly test pipe to connect the downpipe to the AMS catback that came with the car. It worked out pretty well; the car is obviously louder, but I don't appear to have any leaks, and the car pulls noticably harder early in the RPM band. The one downside: I had to remove both lower braces that would normally run underneath the downpipe, because the whole assembly hangs too low for them (for the front bar, there's just no hope of reattaching it; for the rear bar, it fits, but rattles rather distinctively when the car warms up). Small price to pay, and I can probably weld up a replacement frame that's stiffer later anyway.
Coming in a few days is the last piece of the puzzle for a while: an Innovate LC-1 wideband oxygen sensor. This gives me the last piece of information I'm needing to fine-tune the car: my real air/fuel ratio. Hopefully this will give me what I need to bring the injector duty cycles down a bit at the top end (since I'm quite certain the car is running rich now, but leaning it out without some way to measure it is asking for trouble). Once I have A/F dialed in safely, I can play with timing and boost to make more power.