My car is just about to hit 90,000 miles and it still has the factory suspension all around, all of which is dated 2011. Would new shocks be worthwhile? I've never been in another M so I can't compare if the ride quality and performance has at all deteriorated.

Also, I know many here have opted for aftermarket parts to lower the stance or whatever. Overall I am happy with the ride. It is comfortable at most times and still sporty when I want it to be - just wish we had bigger swaybar options. With that said, I'll probably stick to OEM Infiniti parts.

I haven't looked into the M56x, but I know on the M35x to get all new struts it was like $650 because the fronts (or was it the rears) didn't have an aftermarket OE option so you HAD to go to the dealer. Because of that, you were better off dropping another $200-250 on some coilovers. That may be the case here.

I'm at 85kmi myself and while I don't feel or hear anything normally...over certain roads (for example, cobblestone which I drove over yesterday), the car suspension (pretty sure the struts themselves) rattled like a MOFO. I would say 85-100kmi is about as long as I'd expect the stock suspension to be good...but it depends on the roads the car is driven on, how aggressively, etc. I drive my car hard cause I live in the boonies and have wide open road to push the limits on.

I myself will probably just go with BC coilovers when the times comes for a refresh along with new stabilizer links, bushings and if BC doesn't include them, strut mounts.

You probably will not notice much change to the ride after installing new struts unless they are completely shot. But the suspensions effectiveness at keeping the tires planted on the road and how it behaves when you need to make sudden evasive maneuvers will be improved. Tire wear can also be favorably impacted by properly functioning struts. That said if you have no issues now they are probably fine to keep running. I would personally change them out sometime after 65k with 100K being the limit.