Steve Millen has been building everything from sports cars to SUVs for over a decade, and his attention to detail has resulted in many extraordinary vehicles. Such is the case with Stillen's Yukon Denali XL GTS.

The fully loaded GTS we tested started life as a factory-fresh 4WD Yukon XL. A 112cfm Magnacharger was added to the 5.3-liter Vortec V-8, as were custom extrude-honed exhaust manifolds, a high-capacity transmission cooler, and a dual exhaust with 4-inch-diameter tips.

The modifications are good for a dyno-proven 400 horsepower and 400 pound-feet of twist. At the track, the GTS impressed us with a 0-to-60-mph pull in 6.8 seconds, effectively shaving 1.1 second off the stock time, and a quarter-mile run of 14.97 at 88.1 mph bests the factory unit by 0.92 seconds and 1.8 mph. On the street, initial acceleration is impressive, but it's the increased passing power that gets your attention, feeling more like a Corvette than a full-size SUV.

Underpinnings remain stock, with the exception of larger anti-roll bars and massive front-mounted six-piston AP Racing brakes (with 14-inch slotted and cross-drilled rotors). In the rear, the stock units were cross-drilled for quicker heat dissipation. Gone is the usual SpongeBob-feeling brake pedal, now made firm and communicative through its 126-foot 60-to-0 emergency brake test--two feet shorter than stock.

The larger bars and 20-inch Antera 323 wheels shod in Nitto NT-404 rubber make this XL handle like a vehicle half its size, posting a slalom run of 57.7 mph, 4.3 mph faster than a factory-fresh model we previously tested. We found turn-in to be crisp and body roll greatly reduced over the stock offering. More important, the ride is wonderfully compliant over rough pavement.