There should be a setting for half/double/normal tempo in the parameter box. I sent this as a feature request to Apple, but it has not made it into 9. Hope they do add it in a minor update as it can't be that hard to program. It would open up quick experimental avenues for apple loops.
Don't say if you're using Apple Loops, nor what version of Logic. But, you can do this by using the resize pointer - go to bottom right corner and option/drag loop to half its length. If Apple Loop, deselect the follow tempo box in the region parameters first. You'll be asked to make a new file. Alternately you can create a new audio file (non-Apple Loop) before you change region length.
Your pre-selected time-stretch algorithm can affect quality. It's under Arrange menu audio.