Although my progress has been set back many months, it seems I can keep working on it. Trying to go between GIMP and CS5 accidentally lost me a lot of work and I had to go back and re-do what I'd already done, but based on the comment from the other thread somebody suggested where I can change some settings in the color options of CS5 and it seems to work.
I really dislike CS5 compared to PS 5.1. The options are all hidden and jumbled around, it's not as simple and user friendly, and they changed how the program handles layer blending (defaulting to 8-bit? Blech!!) and down-sampling. Case in point it was force-dithering every time. There's an option under color settings that I think was the culprit: "Use Dither (8-bit/channel images)?" which among other settings I changed. It now seems to allow me to do what I want. You have to hit "more options" to even see it as checkbox to uncheck, which is lame to the extreme...
Anyways, I'm trying to re-acquaint myself with where I left off. I need to do some more work on details and keep tweaking the weathering (exhaust and stains are much better now!) but what's really annoying is my guess-work with how I want to adjust the bumpmat.txt file. I've been trying out a lot of settings. Also toying with levels on the specmap. Making exhausts reflect the least, the oil leaks reflect more, the default camo reflect medium, and the field-sprayed camo to reflect more (as if it were fresher, not dulled by time and the elements). It's a bit of a guessing game as well. Sometimes it doesn't seem to turn out how I imagine it will. Oh well. Progress is progress.
