Great question indeed - I don't know if I have the cure all solution you all are wanting but I'll tell ya what I can.
It's true that the water temperatures near coral reefs does fluctuate more than you might expect. As mentioned, surface to depth temperature gradiants exist for sure, as do seasonal, and day to night swings. Tides and currents will also bring in cooler or warmer water.
The thing to remember though is that most all of these "natural" temperature shifts occur gradually - not normally very fast at all.
Obviously to us all is the water nearer the surface will heat up from the sunlight. The closer the reef is to the equator will affect this rate as well - how much - I have no idea.
Most everything I've read states that you find the most mixed species of corals in waters between 70-78 F. Areas of the ocean that are normally below 60 F or above 85 F - few corals are found.
So ..... still haven't answered the question.
As stated - as long as your temperature swings are gradual increases and decreases - I don't believe you'll have any major problems.
Lighting, pumps, powerheads, amount of live rock/substrate, evaporation rate at the water surface, w or w/o sump, all play a part in either cooling or heating the tank.
An empty glass box of water, no rock, no sand, nothing but lighting, some circulation, a heater and water will swing faster from day to night, compared to a tank loaded to the hilt with live rock, 4-6 inch DSB, fish, inverts, etc. There's more mass there to hold the heat in once it get's to a given temperature.
My 75 has two heaters in the sump - and so far it is holding steady at about 78F. Summer poses more trouble keeping it cool.
Anyways ... a steady temp is ideal.
2-3 degree swings are good - you're fine.
4-6 degrees is not uncommon - but watch your fish and corals close.
10 degree swings are not recommended at all, and you would be wise to explore the reasons and try to correct.
