On medicine/surgery rotations, you actually have a chance to help with your work ethic, good H/P's, presentations, write notes, or in other little ways. On a radiology rotation, you are just taking up space and not contributing squat. You will have no idea what you are looking at, outside of routine CXRs. So you don't really have much of a chance to impress. Sure, maybe you will be the rare student with the very keen eye that is very impressive, but more likely you will not have a clue what you are looking at, and come off like an idiot with no clue every time you are asked a question. You have much more chance to hurt yourself than to help yourself, IMO.
You will probably rotate through the department the entire month, so you will probably not work with the same attending for more than 2 days consecutively, meaning a letter is probably worthless and you will not be remembered come interview season.
The ONLY reason that an away would be worthwhile is if you are trying to overcome a geographic barrier (ie. lived in Cali your entire life, trying to move to Boston). If you grew up or went to med school in the same region as your dream residency, I wouldn't even bother. If your numbers are as strong as the OP's, you will get interviews in every region.
---This only applies to a general radiology rotation. I think IR would be helpful, as would a research away rotation.