Give me a minute I need to google the MK.10 PA ... .... ... ... ...
THEY DID THE EQUIVALENT OF 10 000 YEARS OF RESEARCH BY MARS ITSELF IN FUCKING 5 DECADES!!!!!
Sweet Star-child is all of our research so OP, because if yes than I guess we should do more after getting rest of faith actions and med to 5
Do keep in mind, it's not "10,000 Years of Research by the entirety of Mars"
It's "10,000 Years as a side gig by one dude, based on a design that was already largely perfected as something he did on the side in the face of his Main Job, which was to somehow improve on the base Astartes formula." The Mark VIII was functionally optimal for Astartes use, in that it had no obvious vulnerabilities and excellent performance with no compromises in mobility.
The Mark X's main expansion on that is being modular, in that the same base chassis can be easily adjusted for a given Astartes' role or intended mission. You don't need a separate suit if you want to be a sneeki beeki, or if you want to get in the vanguard and hit things with sword.
Functionally, Astartes Power Armor reached peak performance with the Mark IV, which was just pre-Heresy, but had plenty of obvious vulnerabilities, like exposed power transfer cables (Even if armored). Mark VII partially closed the vulnerabilities that Mark IV had while improving protection (While Mark V and VI were "Kitbashed designs for use for Anti-Astartes combat on low logistics and specialist skirmisher variations respectively"), and Mark VIII reached peak optimization in that it maximized protection without compromises elsewhere, aside from being a bit more tricky to mass produce.