Out of curiosity, did the machine stay running the whole time?
It has been suggested that the efi only reads barometric pressure at key-on events, meaning if you drive all the way from 3000 to 8000 without stopping, it might not have adjusted. This doesn't make sense to me, as the MAP (Manifold ABSOLUTE pressure) is a pretty important input that is constantly used...not to mention the O2 also helping lean or rich...but heck, it's worth a try.
You can also pop the airbox lid when it's safe to do so...it'll help. It'll sound like satan belching, though your cab will probably keep it out.
Also, run the thinnest oil in the range. It's an old drag racing trick to use thinner/thicker ATF to fine tune stall speed with a torque converter, and I think if you could get an extra few hundred rpm at stall, I'd help a lot. I don't know if it actually does anything for the pioneer, but it can't hurt.
There is also a thread with a guy working with a power vision 3, and I've always been curious what an extra degree or 3 of timing might do at high elevations.