@ApolloBoy, Mickcris, Lilralphie
There is no reason not work in 5200, since it is originally equipped with GTIA, but I'll try to remove the doubt.
I'm not sure, but I suppose about US$50-60.
All color registers are eight-bit, so yes, GR.10 also have 16 luminances.
1. No, the least significant bit of luminance isn't on/off, athough could be if you can propose a proper, free bit in some GTIA register.
2. I don't know but if you can point some test program, I can check it out.
3. There is no problem with the positioning GTIA-mode pixels relative to hi-res graphics. As above - give a test.
It isn't a stupid question. It will work, but the pinout of GTIA is different than FGTIA and there are significant differences in the TRIGs inputs handling, so different CPLD firmware and an adapter (or another PCB version) would be needed.