Significant downgrade in performance from your .270 Winchester. The 6.8 SPC throws 110-115gr .277" bullets at about 2600-2700 fps, while the .270 Win will throw 130gr bullets to 3000-3100 fps.
Keep in mind the point of the 6.8 SPC is to improve hitting power within the confines of an AR-15 based rifle. This it does, but I believe it's at best a good match to .30-30 power level with the better downrange ballistics of pointed bullets.
Edited: Dang, need to type faster next time!
NRA Master, Highpower Rifle, Across-the-Course
NRA Expert, Highpower Rifle, Mid-Range Prone