I'm using R1800 (same inks as R800) and didn't notice any colour shifts in photos printed when changing cartridges. I didn't do the formal test like yours, but will try it from now on to see how things develop (there are some cartridges that aproch the end...).
Did you try printing without colour management - just using colour controls (Brightness +1, everything else -1) or special photo modes in printer driver? After several tests, it seems to me that black&white printing is better done through driver (Colour controls or Photo Enhance B&W Neutral), bypassing PS colour management. Blacks are much more neutral and there's more detail visible in print, which is lost when printed through PS colour management. For colour printing I'm using Epson canned profiles for specific media and it works for me quite well (soft-proofed screen to print match well).