We have to do either a medicine/surgery/peds sub-I. I did an IM sub-I in July of MS4 and it was a great opportunity for me to get a letter from someone who could see me in more of an experienced, leadership capacity - and this was remarked on during a number of my interviews. Speaking just for my interests, I was deciding between Psych, IM, and Med-Psych at the time, so the experience came early enough to afford me a strong letter, more insight into what I wanted out of my career, and flexibility with what my application would look like. I learned a lot, and it was one of the first times I really felt somewhat competent - one of my favorite rotations in medical school.
Speaking to your situation, my question for you is do you think you have a strong non-Psych letter in the bag? With your Step 1 score, I'm not sure whether an away Psych rotation or a strong IM letter would help more in demonstrating to programs you'll be a capable resident. Re: aways, this was not something anybody at my program has done in recent years for Psych, maybe that just speaks to the number of rotations we have available and the strength of our program, but on my interview trail I saw it was quite common for applicants from programs that have smaller Psychiatry programs and I think if you fall in this boat doing an away would serve you quite well. This isn't necessarily like Ortho or EM where you need to go to several places, even one away could be fine if it's at a place you think you really want to go to but may be overlooked at because of your numbers. How many were you thinking of doing? I'm in the match right now so this is all n=1, I'd put more value into the opinions of your advisors and folks on here who have some experience in evaluating applicants.