Several thoughts, which could be used alone or in any combination:
1.) Have you thought of so-called "swamp coolers" (evaporative coolers)?
These devices work on the principle of evaporative cooling. There are 2 basic types: Direct cooling and indirect cooling. The direct cooling units are VERY simple and easy, but may result in air that is too humid to be comfortable (even though it is much cooler). The indirect is more expensive and hard (but not nearly as expensive as conventional AC), and results in drier air. Some systems use both methods: pre-cool the air via the indirect method, then run the cooled air through a direct cooler. The result is cooler and drier air than either method alone. All methods work best with very dry air (which it appears you have).
DIRECT method: Dry outside (or even inside) air is drawn through a wet screen or filter. The cooled, moist air is pumped directly inside. This method can be as simple as throwing a wet towel over a fan.
INDIRECT method: Dry outside air runs through a wet screen or filter, to get cooled, and then across a heat exchanger which. The dry air picks up moisture which cools the unit. Inside air blown into the opposite chamber of the heat exchanger is cooled, but picks up no moisture. Even this relatively complicated method is easier and very much cheaper to build (and operate) than an AC unit.
2.) The simplest way to keep things cool is to deny the sun entrance.
I had a home with a large skylight in the kitchen. Because I lived in a very temperate part of the northwestern U.S., I had no AC. In the summer I would block off the skylight totally with a very light custom-cut board lined with aluminum foil; then take it down in the fall. It made a huge difference (probably 5C in the kitchen).
You could do something similar, using greenhouse shade cloth, chimney flashing, or many other items that run the gamut of the aesthetics/functionality tradeoff.
3.) Venting the hottest air (from the top) and replacing it with cooler air will help; or you could leave that air alone and recirc the cooler lower air, to make it even cooler. In either case, forcing the air through deeply buried pipes would cool it.
The temperature of the earth, almost anywhere on Earth you would want to live, is roughly in the neighborhood of 50F (10C). You just have to dig down deep enough to access that temperature. All the desert critters know this: that is why they stay underground during the day. Even if you only want to go down two meters, you should be able to access constant temperatures of lower than 15C. Then you bury some pipes and force the air through them. The air heats the dirt, but that dirt is cooled by the surrounding dirt. Basically you are using mother earth as a heat sink.
Actually your AC is much more efficient than that because it is a heat pump, not a direct conversion of electrical watts in to BTU of heat moved per hour. If you know your SEER rating, you can just divide the BTU/h by SEER to get Watts.
An SEER of 10 is very common so that would mean each AC needs 800 W to move 8000 BTU per hour.
http://en.wikipedia.org/wiki/Seasonal_energy_efficiency_ratio
BTW, it's good to keep your units straight between energy and power (which is the rate that energy is being used).
Watt-hours and BTU are energy.
Watts and BTU/hour are power.
And your AC is not 8000 BTU, it's 8000 BTU/hour.
Best Answer
Rather than focussing on how it's done (inverter drive) focus on what it does (variable speed compressor)
Essentially, the variable speed allows the compressor speed to match the cooling load, so rather than a fixed speed compressor starting, cooling, stopping, starting, cooling, stopping a variable speed model can simply adjust its speed to match the cooling load, while running continuously (until there is no cooling load, or one below the minimum speed it can run efficiently, anyway.)
That has several potential benefits - one is that starting the compressor motor takes considerably more power than running it - so many starts and stops add up - this (alone) might be what someone who says "they only save power over 8 hours or more" is speaking of. However, I think any such statement is far too general to pay much attention to, given the variations in individual units (I've been considering mini-split heat pumps, and have read a lot of data sheets - some are surprisingly different even for closely related models made by the same factory...)
Additional benefits are that rather than the system blowing air that's cold, and then stopping, it blows air that's cool, all the time. Among other things, this improves dehumidification performance and is also usually more comfortable.
You state that you use your air conditioning less than 8 hours per day - but how many days a year? It is true that if the initial cost of something you use rarely is much higher then the payback for any more efficiency (from a unit costing more) is very long (and may exceed the useful life of the product, in some cases.) However, that is not something that is amenable to analysis from afar, and will vary with the initial cost, the energy cost, the use, the climate...
If your cooling needs are limited, and the cost differential is high, just look for the best efficiency among the lower cost type units, and don't worry too much about "the future of HVAC" if having it does not make personal economic sense for you. Something better may be invented before this air conditioner wears out...
Of course, if you are buying 9 of the things, you should at least consider a central system. But that may not pay off in a reasonable time, either.