When I had a 655, I had called Taylor about using mediums, and they said that's fine but recommended going down a full step (to D). I did so and the tension, according to my finely calibrated fingeromometer, reported that mediums tuned down to D felt about the same as the light gauge set tuned at concert pitch. But I preferred the tone with the mediums in D. Yet on my RT12, I love the tone as is so left the lights on and kept it at concert. So your best bet is to try it and see if you like it.
Edward