I am checking into Dolphin for Princess next week and it has been interesting watching the rates. I got my lowest AP rate in late September (280/night including fees) and suddenly these past few days it dropped a few more bucks. Usually race weekends are so busy it surprised me there's still availability and at such a low rate. Yesterday even the regular refundable rate was only 300/night including fees for Thursday check-in, probably the most popular day for those checking in for the challenge (10k + half on Saturday/Sunday). More days than not the rates have been significantly higher (even 100/night more), but the low rates pop up for those that check regularly like me, even last minute.
I actually decided to book through the Amex Travel portal since I got an Amex plat. The S/D are part of their Hotel Collection which comes with a $200 annual statement credit if you stay 2+ nights and a $100 resort credit that it sounds like people have been successful in just applying towards the invoice (resort fees) rather than towards dining/spa/etc. The Amex Travel price was about $170 more total than my AP rate, but the $200 credit will cover that and I have no other stays this year where I would be able to use it. Plus, the $100 credit on top of that would definitely make it worthwhile.
The Amex Travel benefits include Marriott Gold status and a room upgrade when available, now I know that even the top Marriott status members have a tough time getting upgrades at the S/D but in my digging it does seem like the Amex Travel status often is prioritized over regular Marriott status, so I will definitely report back if I have any luck. All room types still being available and heavily discounted is a good sign. I will also request the 2pm check out, which also seems to be rare, but we shall see. I linked my Marriott number in the Amex portal and it immediately showed up in my Marriott account so I'm definitely confident I'll get both the Marriott points and the 5x MR points for booking via portal. I have a free night promo with each stay too that I'm hoping this qualifies for even though I didn't book direct.
Not trying to sound like an Amex ad

But hoping this works out well so I can report back if it's worth booking via portal or not, since it seems like hardly anyone gets upgraded on Marriott status alone.