The XTS 3.6 V-6 is rated @304 HP while the same engine in the CTS is rated @ 318.
Where does the extra power come from?
I think the XTS could use about 350 HP and mix very well in the Euro/Asian crowd of premium sedans.
I believe the difference comes from the exhaust plumbing. Because the engine is mounted transversly in the XTS it isn't able to use the same freer flowing exhaust that the CTS has.
Keep in mind that the CTS too was rated @ 304 hp before the 2012 model year.
On second thought it looks like the increased HP in the CTS is related more to intake changes. Not sure then why the XTS doesn't benefit from the bump.
The following was from Car & Driver:
The revised direct-injection 3.6-liter V-6 now puts out 318 hp and 275 lb-ft of torque, thanks mostly to intake changes, including larger valves, revised ports, and a tweaked camshaft that keeps the intake valves open longer. (The same engine makes 323 hp in the updated 2012 Chevrolet Camaro.) Compared to last year’s 3.6-liter V-6, this year’s iteration is nearly 21 pounds lighter; 13 pounds were saved by integrating the heads and exhaust manifolds, 2 pounds from a lighter-weight front accessory-drive cover and new connecting rods, and 5.5 came out from using a plastic intake manifold in place of last year’s aluminum piece. The base 270-hp, 3.0-liter V-6 carries over unchanged, and can still be mated to both a six-speed automatic and six-speed manual. The manual is no longer available with the 3.6-liter, however.
When I had my 1995 SLS I also had the service manual. Looking at everything on the two available engines SLS "275 hp" and STS "300 hp but with a lower torque", the only difference were stronger valve springs in the 300 hp motor. Perhaps that the case here between the CTS and XTS where the XTS has a slightly lower HP but perhaps a slightly higher torque.