First off, the Sheraton Maui is a beautiful resort and I loved every minute of being there. I think no matter what room you have, you will have an amazing stay at the resort. And don't forget to visit Mama's Fish House while on Maui for by far the best fish dining experience on the island.
As for room upgrades, I found the hotel staff very accomodating. I am SPG Gold and actually booked my stay through Costco, so prepaid (at a much lower rate than Starwoods' advertised rates) and was not earning SPG points. The room I booked and paid for was a Mountainview Room. When I arrived at the hotel, I gave my SPG Gold card to the front desk and was upgraded to an Ocean Front room. Not a deluxe or suite, but we thoroughly enjoyed the Ocean Front and the great breeze on the balcony. I can imagine that if you are Platinum, you will be upgraded to a much better room. Unless the hotel is completely sold out, I think your chances of being upgraded are extremely good. The hotel is really top notch in service and taking care of their guests.
Enjoy your stay!