From what I found out from our G7 certified rep is that unless you're printing from CMYK or ProPhotoRGB formats, you will almost NOT see any Orange or Green used regardless of the colors in the image. The colors will be used during cleaning which is why you'll see the levels go down, but not used in RGB format.
Also, what Mark stated is also true on printing usage to choose which model would be best for your purpose.
Depending on the type of 17x22 paper you print on, both will work fairly well. The feeding in the P5000 is fairly nice though as you put the sheet in and have it read the paper before it takes it to print on. The P800 will sit in the feeder and accept once the printer starts the printing. If it's fine art on the P800, you have to make sure it's loaded correctly so the printer can read and accept to print. I haven't done fine art sheets on the P5000 to say for sure yet, but the feeding process is much easier on it than it was on the 4900.
A couple of clarifications are in order here:
Firstly, the SCP5000 does use Orange and Green ink for prints in various working spaces, but not much. For example, my printer/profile verification test chart is in Lab space and it does use small amounts of Orange and Green, but less than the other colours. They are the last cartridges to be emptied, but they get used.
Secondly, for the P800, if you are using the top feed, you drop the sheet into it, so it can only know whether it feeds itself properly once it needs to pull the paper through for printing; however, I have hardly had misfeeds from this loading mechanism. When using the Front Fine Art feed, yes, you know before you do any printing whether it is skewed or not.
