You can get various flavors of Screamin' Eagle cams for the Sportster engines, ranging from mild to wild.
They range from the lower lift/flowing .497 cams (.497 lift intake/exhaust), to the .536 lift, the .551 lift (also known as the "E" cams), and the .575 lift model.
The .497 and .536 lift Screamin' Eagle cams can be used with the stock pushrods. lifters and rockers without a problem. The higher lift/flowing .551 and .575 cams are 'bolt in' for 2004+ engines, but if used on 2003 and earlier engines you will need different springs and 00+ pinion gear.
I'm running the Screamin' Eagle "E" cam (.551 lift) in my 2005 1200C. This cam has the same grind as the cam used in the Buell XB model, and is known as a High Torque grind. Here is some information on the .551 "E" cam from the NRHS Webpage;
The new SE "E" grind is a terrific high-torque grind, relatively narrow with high lift and exceptional valvetrain dynamics. Note that this grind is a bolt-in for 2004 models but in 2003 and earlier models, better springs are required. Also note that it has a .050 smaller base circle than the 2003 and earlier grinds, consider this when calculating ideal pushrod length. This is the same cam grind (but not the same set of cams) that comes in new Buell XB models.
Power gains from these of any other cams will be modest at best without additional headwork. My Harley dealership friend does performance dyno tunes and has told me that the .551 lift "E" cams will give you an additional 8 to 10 horsepower to the rear wheel using stock heads, etc. Power increases to the rear wheel using stock heads and the .497 or .536 lift version would be slightly less.
Of course you can pull much higher horsepower gains with highly modified heads...
NRHS has dyno charts showing around 105 horsepower to the rear wheel in a 1250 engine using Screamin' Eagle .536 cams and highly modified Stage 3 heads.
Guess the bottom line is that cams themselves will give you a horsepower increase, but that horsepower increase will be much more
if you polish and port the heads at the same time.