Austin, Texas - It hasn't been that long since the long-running and popular Jeep Grand Cherokee and its absolutely insane high-performance SRT version were totally revamped: In an effort to continue the momentum, the 2014 models have already been crafted and offer a range of changes both subtle and substantial, for those of you keeping score at home.We got a chance to put both products - the built-for-mortals Grand Cherokee, including a new diesel-powered option, and the totally nutty 470-horsepower racing chariot version - to the test in the Texas capital, with a very exciting morning spent in the latter on the all-new Circuit of the Americas Formula 1 race track, as well as an afternoon on a wonderfully scary rock course up in the Hill Country.Why re-do a good SUV so soon? Jeep managers say they're anxious to continue the 207 percent growth in sales they've had in the last couple of years, as well as pushing the more luxurious Jeep into markets in Europe and around the world.They've also been able to increase efficiencies in every iteration of the Grand Cherokee family with new eight-speed transmissions (part of a $374 million investment at plants in Indiana), engines that deactivate half of their cylinders for fuel savings at cruising speeds and ... that new diesel option.The first diesel in a long time for the brand, the 3.0-liter EcoDiesel V6 is estimated to generate up to 30 mpg on the highway. We put that to the test while making the long trip back to town from the rock ranch and ... yes, indeed, you should be able to achieve or exceed that figure without too much effort. You'll be immediately reminded that this is an SUV-worthy diesel, with 240 horsepower and 420-trailer-hauling-lb.-ft. of torque - it lacks the buttery subtlety you'll find in diesels in the VW Passat, but it's still relatively quiet and shares a similar urea-fluid-based exhaust scrubbing system to practically eliminate diesel smells (and much of the CO2). You can also get a Limited model with a diesel for $40,295 or add the diesel as an option to the Overland and Summit models for $4,500 to $5,000.Otherwise, it's the efficient 3.6-liter Pentastar V6 (good for 25 mpg or higher) or a 22-mpg 5.7-liter Hemi V8, both showing 2 mpg increases. Mix those options together and you can find models ranging from a $30,795 Laredo 4x4 to a $51,000-plus Summit 4x4.And the Grand Cherokee itself? The 2014 changes are, for the most part, subtle enough not to be extreme on the outside, including LED headlamps, a nice seven-slot wire grille on the Summit model and a painted lower lip, as well as larger tail lamps.Inside, the updates are more obvious, including an infinitely adaptable 7-inch display in the instrument cluster that can toggle between fuel economy, 4WD information and the optional adaptive cruise control. There's also an overall color-coding scheme specific to each model, including a chrome and copper, Grand Canyon-inspired look or warmer, Morocco-inspired earth tones.And yes, even the fanciest Jeep of them all still does very Jeepish things, when required: My Hemi-powered model, with the full Selec-Terrain system and the 11.3-inch air suspension, literally crawled up and down a 45-degree, 300-foot-tall rock slope, without breaking a sweat. You won't ever do that in real life, but you can if you need to.The more thrilling display of Jeep's swagger is the even-more intense SRT, the 470-horsepower, 160-mph-capable, 4WD behemoth that answers questions perhaps you'd never thought of asking.SRT also gets the eight-speed transmission and a new Selec-Track system that can send more of the power to the rear wheels for even faster performance. There's even a launch control system to fully optimize takeoffs, plus a better spoiler and big 20-inch wheels.I'd driven an SRT on public roads last year but had never had the opportunity to fully wind it out on a race track: The amazing, 3.4-mile-long COTA course, complete with 20 turns, was indeed a good environment to let it all hang out - and even touch 127 mph on a long, long straight at the back.For a substantial vehicle, the SRT does indeed hang in there - certain laws of physics can't be avoided with 5,150 upright pounds of metal, but the damned thing will do the quarter mile in the mid-13-second range, and the massive Brembo brakes, when not smoking from track use, will stop you from 60 mph in 116 feet. Steering feel is wildly accurate and the SRT had an impressively composed feel on the very tight corners, as crazy as that sounds. The roaring, full-throttle exhaust note is worth the price of admission. Cost? Starting at just over $63,000, and fewer than 4,000 will be available for the U.S. market, so snap 'em up while you can.