What a difference a V makes. Added to Cadillac's XLR sports car it means near-Mercedes-Benz-SL55-AMG performance at an SL500 price. Externally the XLR-V tip-off is up front and not all that aggressive, in rather British-looking polished wire mesh grilles. There are unique 19-in. alloy wheels and four shiny exhaust tips, but what matters is the hood bulge. That bump was necessitated by the supercharger atop the twin cam, 32-valve aluminum Northstar V-8. For the sake of block durability, Caddy decreases the bores to give a displacement of 4.4 liters, down from the normally-aspirated Northstar's 4.6. The cylinder heads are new, as are the heavy-duty pistons and stronger connecting rods. The compression ratio drops from 10.5:1 to 9.1:1. This isn't just the standard engine with a blower plunked on top, but a well-executed piece of reengineering. Cadillac has beautifully integrated the blower onto what is almost a new engine. As an example, the intercooler is internal, four finned tubes lurking under the engine cover that look like they came off the back of a fridge. The intake was tuned for maximum flow and what Cadillac considers just the right sounds, though arguably they tuned out too much supercharger whine. The muffler hums an appropriate Cadillac sound, but at full throttle converts to straight-through for better flow and a nastier note. This supercharged package pumps out 443 horsepower at 6400 rpm and 414 lb.-ft. of torque at 3900, with 90 percent of the latter on tap from 2200 to 6000 rpm. With its Corvette heritage, the XLR-V has a rear-mounted transaxle, but with a new 6-speed automatic that can be shifted manually and is programmed to minimize shifting when cornering. Those supercharged Northstar numbers are shy of the SL55 AMG's 493 bhp and 516 lb.-ft. of torque, but the Cadillac's curb weight of 3870 lb. is some 400 less than that of the Mercedes. The result is a pair of supercharged luxury sports cars that get to 60 mph in the mid-4-second range. Paired with the added V power are suspension upgrades. The front anti-roll bar is larger, and they've added one out back. The XLR-V's Magnetic Ride Control is firmer and the StabiliTrak has four positions of firmness and higher performance thresholds. With the 19-in. wheels come 235/45R-19 front, 255/40R-19 rear Pirelli run-flat tires. Rotors are bumped to 13.4-in. front and 13.0-in. rear and cross-drilled, giving the Cadillac the brake hardware of a Z51 Corvette.