I just changed my headlights on my 2009 CTS. WOW what a cluster [email protected]%&. I installed OEM's headlights and had to adjust mine. Good news is you can access the horizontal adjustment screw from the top of the headlight assy. All I had to do is remove the 6 plastic screws/buttons and the rubber gasket then take off the engine shroud to see the access holes in the metal frame to get at the 10mm hex screw on the top of the lights with a 10" extension.
P.S. the local Chevy dealer quoted me 1.8 hours per light @ $95.00 bucks a hour. I took me 1.5 hours. Not that tuff of a job.