What I've heard about Norman's is that quality/experience has detracted a bit of late because Norman spends most of his time at his new(er) LA place.
I've had several great meals at various Mark's restaurants, but not in many years, so I can vouch for the current state of this upscale place.
I really do not know much else about Coral Gables because it is always far from where I am staying (see below), but there is an upscale Argentiniean steak house, Graziano's, down there that has got great reviews. They reportedly import some kinda special wood from Argentina for their asador or roaster.
The sense I get from other foodie forums is that the upscale places in S. Florida are not great, who knows, but IMO, there is so much other places (i.e., "ethnic") that it is hard to NOT eat well down there.
How far is Key Largo from the rest of Miami? As you can probably tell from my posts, we were staying in Broward, which has great chow options. Still, I gotta bet that will be too much of a schlep from the Keys.
Rob