Our shop has a c14000 and a c7100. Both were installed late last year to replace our c6100 and c4080.
I think KM techs even at a higher-up level have had a heck of a time adjusting to these machines. Both of our engines were riddled with bugs and it got to the point where we ultimately had to have KM replace our c14000 with a newer model. Given that our newer unit is night-and-day BETTER than our previous unit, I would honestly keep escalating your case until they resolve whatever fundamental issues your unit is having. In our situation, it was constant banding, hazy and inconsistent solids, consistent fuser faults, a random image stutter that made smooth sheets look like they had a laid texture, prematurely wearing parts, and an awesome dark line that refused to leave the center of the sheet parallel to the paper path. It took five months, but it was worth it.
My reasoning for continuing your escalation is that the c7100 is not a true successor to the c6100, at least in practice. It very much looks and acts like our previous c4080 and still has some jarring bugs that our techs are working out--namely insanely out-of-whack density balance issues and a random post-rip color shift that dramatically changes the color of a project over the first 20 sheets of a production run. It is a good machine in my mind for chugging along on lengthy, non-color-critical runs such as workbooks, flyers, or basic envelopes. To this day I would not trust it for our color-critical customers and it can do absolutely goofy stuff to certain types of media that the c14000 runs completely fine.
Our shop also had those retrofit parts you described installed on our previous c14000. To my knowledge they did not resolve much of anything past the first few days. I'm sure they have improved the quality of some machines, but in my mind they were some plastic parts that delayed the inevitable.
Again, our new c14000 is running ridiculously well as of this writing. Keep documenting your case, run test sheets and datestamp them, and keep a running service log of your downtime independent of your service team. KM doesn't make maintenance money unless you make money, and there will come a tipping point where it is more economical to send in a new unit instead of hammering away at the old one.