How do you know by replacing RDM with a COR and making your SMN the mainhealer is not going to improve exp/hr further? To show that DD/buffing SMN works better than mainhealing SMN you need to replace RDM with a COR and mainheal on your SMN to see if there is any significant difference in exp/hr:
Party setup A - DD, DD, DD, BRD, SMN , RDM
Party setup B - DD, DD, DD, BRD, COR, SMN (keep DD and BRD, replace RDM with a COR)
I can assure you, if setup A nets your party 25k/hr, setup B is going to do even better, provided you are not a retarded SMN who burned all MP on BPs.