It's going well so far. We are 3 sessions in and have not had any major conflicts yet.
We kept the additional backgrounds etc close to the playtest examples in build and made them before making characters, to prevent anyone coming up with something fancy tailored for just their PC. We also added a dwarf subrace (steppe dwarfs) as we have several dwarfs in the group (everyone plays 2 PCs) we wanted more variety and added human subraces (cultures) relevant for the AP.
As we have a bard, we just have spells for bards work a bit differently, mainly they have to be performed in some way or the other.
We aren't going to modify monsters not in the playtest description. I'll decide on the fly how they'll adapt, if needed. But we do that a lot, saves time.
I think it is important to know beforehand which changes, if any, need to be made in the game world. We picked an AP we originally wanted to fastplay (in a 4 day weeked) but proved too complex and sandboxy to do so, set in a world of its own. It reduces the chances of getting in conflict with source material.
We haven't run into any rules related obstacles yet, and we are all aware that we may need to make some adaptions along the way.
It's not too much work, and it's been fun to create additional material for a basic set of rules without having to worry how it may interfere with older rules or how it may be exploited - especially as it's not a houserule/homebrew setting that is going to last any longer than this AP. At least not in our plans
So if you have a little bit of time to make additional content and have a group that's into the story more than minmaxing and ready to make changes along the way, it's well worth it in my opinion. You really all need to be on the same page, though - it is still an extended playtest no matter the added content.