1156s in the inner sockets will be too bright. From my understanding, the inners are only running lights, with no brake/turn function. With the lights on, they will be as bright as the outers are when the brakes/turns are on.
I had this issue on a non-RS I just built, and found on TC that it isnt an uncommon problem. Had 1157s outer, and 1156s in the other two. I had never even heard of 1155s, but they give the correct brightness on the inner taillights.