I'll post a full report once our stay is complete, but we checked in at night (EK arrives at ~4pm + time for immigration + an hour or so to the resort by car) and check in is done in the room, so no real opportunity for upgrade negotiation. The next day, we asked about a Manor House Suite and were offered the opportunity to "upgrade."
We got to the room and were surprised that we didn't feel like it was an upgrade at all. The St. Regis Suite was beachfront (2nd floor) and had a perfect view of the beach/ocean. The Manor House Suite is in the main building on the second floor and has a very obstructed view of the sea + the noise of the main building including pool, restaurant, etc. Also, the St. Regis Suite is a true suite with 2 separate rooms, whereas the Manor House is more like an oversize room/junior suite. Lots of bathroom floor real estate is taken up with the 2 massage beds, and at +30% price for every massage (x2 people...) we'd go to the spa anyway. The only benefits IMO are that it's closer to the pool and breakfast + the massage beds if you're going get lots of massages and don't mind paying the premium.
So to the hotel's credit, they allowed us to move again--this time to the St. Regis "Grand" Suite, which I think is just the ground floor of the St. Regis Suites, so the floor plan is similar, but it has a huge ground floor terrace that leads out onto the beach. We're very happy with the room and 5X upgrade on a C&P rate. So moral of the story is that, at least in our opinion, both the St Regis Suite and St Regis Grand Suite are preferable to the Manor House -- would depend on what you value most. The other moral of the story is they seem to be willing to bend over backwards for plat guests to get you in a room you like.
Other than room commentary, staff has generally been excellent. Our butler is absolutely fantastic--he's learned preferences in just a couple days and continues to adjust service tailored to our preferences--and another member of staff (I can't tell if he's concierge or management) has really gone above and beyond with our digital camera which broke mid-trip. I just asked if the hotel had any tech staff that might be able to take a look. He offered to take it to a camera shop on his way home, and when I said I appreciated it, but that really we wanted a quick opinion and might prefer to just buy another camera so we don't miss many pictures on the trip, he said if they can't fix it quickly, he'll just bring us his camera which we can use as long as we're here. Absolutely amazing!
The service is a bit spotty at times from "second level" staff like housekeeping, turn down, towel refreshers, room service clearing, etc. but not enough to be much of a bother. We did have an uninvited guest in the room last night (a mouse) while we were enjoying room service, which the staff helped us get out. It almost certainly entered while housekeeping was cleaning the room, as I've noticed they like to keep the door open while cleaning the room. I've noticed this at other hotels as well and can't for the life of me figure out why you'd leave the door wide open in a lively tropical location... Anyhow, the butlers assured us that our door would be closed during cleaning today...