I would also make sure your technique is not the real culprit of your pain. The serve is a very complex series of motions and doing something awkward can put a lot of repetitive stress on joints.
While the 315 is a very comfortable racquet and very arm friendly, it is not a cure (no matter what the sticker on it says) for arm and shoulder pains. The best it can do is only make the racquet a little less jarring on impact when compared to most other modern racquets.
3X PK Ki5 315 ::: 4X PSLGT and 1X PSL ::: 2X PSTGT and 1X PST
MCS mains and PPA crosses