I suck at log analysis so I won’t even attempt that and will let the pros handle the deep dives! But Resto Shaman healing is basically built on two things – Deep Healing (Mastery) and cooldowns. If you are not making full use of CDs like HTT, SLT, and Ascendence, your healing numbers will be low. If you are finding that your raid is healed up before you really get a chance to do so, Mastery will be rendered much less valuable and your raid has too many healers. Dropping a healer would let you make use of both core spec strengths.
It sounds like you’ve got the first few bosses on farm, so somebody healing could go DPS or you can just acknowledge that your performance will not be great on those, not because you’re a bad player but because your raid has too many healers. But when you get to the content that you’re progressing on, that’s when you should be shining (not necessarily topping meters, but meters for healing are largely pointless for ranking purposes) in saving people from near death and bringing the whole raid to fuller health very quickly via CDs.
Bottom line is that farm content doesn’t matter performance wise. You want to look at your progression performance and like I said, the pros can do a far better job at log analysis and help there!