By the time you get into bed at the T3 hotel it will be nearly 2pm (if you are lucky) and in my experience you will have to be up at 6.30 for a 8pm flight so is it worth it.
There are later flights to MCT so why don't you, as I have done and will do again in October, get a later flight and have a lie in.
Are you are going to work first thing at Muscat? If not you won't probably be able to check-in to your hotel till 1ish.
SS