Since when does raid leader determine spots based on Ilvl and HPS? That sounds gross. If people are dying on a fight that requires big, constant AoE healing, then yes i'd say its a semi-decent way to see who to switch and when, but that doesnt seem like the case. Best way to raise HPS beside gear, wich is obvious, is to know the encounter, and predict damage. That way you dont fall into the high HPS/High Overhealing category.
Remember how resto mastery works when he snipe heals due to better gear and luck your healing drops a lot . so in soke regard shaman mastery punishes them . if you manage to snipe him he will drop in heals , and no raid leader shouldbwatch on pure hps if people isnt dieing you are doing your job good. Remember shamans heal eficiant just how much is needed wit low overheal
First thing i see is your riptide glyph, and then the amount of healing your riptide does in some of the fights you linked, i really feel riptide should be unglyphed and just used more, especially when you are chain healing as much as you do, I personally think chain heal is garbage, but if you make it work that way then it works ofc. But i think the best 'tip' i could give you from looking at the logs is to use Riptide a lot more, keep it rolling on the tanks as much as possible and use it as a spotheal along with Healing Surges(You rarely use this spell, but the crits are amazing), and if you're gonna chain heal try to pick a target with riptide. Healing rain is also your very best friend in 25 man and you should keep it doing as much as possible, even if its just for the stamina stacking at some points. You are hopefully not stacking spirit to chain heal spam!
You could also look into changing your water shield glyph to a glyph like Totemic Recall, so you can recall your healing streams at the very end and get full mana back, basicly free healing stream totems, i use it and its amazing mana when you'v learned how to.(Works with recalling fire and earth ele too)
Oh yeah and the Riptide glyph could be changed to something like Glyph of Healing Wave or Healing Stream Glyph, both are amazing at some fights. My usual setup is Telluric Currents, Totemic Recall and Healing wave glyphs.
Would'v liked some logs from a fight like Garalon, its kinda hard to judge what you might be doing wrong in the above fights, that your guild seems to also somewhat have en easy time on.
So i guess what im trying to say is that shamans in 25 man is strong in Raid healing(When people are stacked mostly) cause of Healing Rain, And also in their spot healing with Healing surges and just generally fast big heals when people are low. Oh and also their cooldowns are godlike, so the difference between you and the other shaman might aswell just be how you use your cds, important to coordinate and stuff, all that good jazz.