^^^^
As Mat says, Milltek do not recommend mixing their parts with oem parts (I have spoken with Mr Milly many times about this, including this morning!). However they can't control what customers and some of their resellers do and they do need to manufacture both replacement and upgrade components.
Btw, any story you hear about Milltek being manufactured in China is complete and utter bollox! To quote Milltek's boss: "Over my dead body!" would he allow that to happen.