An EASA Part 21J DOA can make it's own decision on whether a change is Major or Minor, based on criteria set out in AMC/GM21A91. If we feel something should be Minor but the regulations point to Major, we can ask EASA to re-classify the change back to Minor (so far I've never managed that!).
EASA do accept some data from manufacturers to support an STC - for example the equipment DDP, System Safety assessment, functional hazard assessment, but in most cases will expect the EASA DOA to at least re-structure the data to an EASA version and expand much of the interpretive data to show specific compliance with regulation AND guidance material. In the US, guidance is just that, and applicants for an FAA STC do not have to show compliance to guidance data. If you don't do this in Europe, your chances of getting an STC approved are seriously diminished.
EASA also like to add loads of extras to an STC - CRIs (certification review items), which are additional requirements where the regulation is deemed to be inadequate - this has been a major issue in glass-cockpit STCs. The FAA also have their version of CRIs, known as issue papers.