I have no idea how much different the ink from the x900 series is, but I had a Epson 4800 for years and used one set of inks that were more than one year old and saw no visual different. I often heard that the colors would shift after a while. If you have your own printer calibration just do a new calibration every 3 months
I'm not concerned with slight color shift, since I reprofile every new roll of paper.
We have an Epson 4800 as well, and had some bad clogging problems with several cartridges that were 6 months expired. Even after numerous shaking, power cleaning, and moistening the rainbow pad.
Once the cartridges were replaced, the problem went away.
This is what I'm concerned about.