@Egregious The math speaks for itself, the linear functions show that the coefficient of healing power is greater for T2 than it is for T2.5. Your data is collected through empiricism and therefore can't be assessed in a theoretical analysis. In other words, casting chain heal with 100% uptime repetitively is merely a linear function. The time it takes to cast a chain heal, and the duration of casting is all you need to produce these numbers. Therefore in theory to acquire the #1 spot for a specific boss requires a T2 set with maximum power. (Maximizing the utter most HPS on a boss is RNG at the end of the day at this level, but a T2 set is a MUST with a team that has coordination among st the healers)
You can logically assess this in any private server given the equal parameters (Three dummies, healing power delta of 200) and by casting on a free object you will find that the HPS is better on T2 as predicted by the mathematics in a theoretical environment. Your data was collected through practice which means sniping, and min-maxing through chain heals was in use. Your 25 second interval isn't going to be the same for other individuals. There are a lot of parameters that deduce at what time your set will be better at performing.
The amount of healers on your team
The individual skill of healers
your uptime
overhealing
The point I brought up about crit was further investigated below, and it has no effect. Applying a crit factor has no probabilistic difference in the outcome. So the math contradicted my hypothesis.
The differences in the sets is therefore the ability to cast the first heal on a set of people in a raid relative to other shamans and healers in your group.
There's a reason why blizzard chose -0.4 seconds and not -0.5.
Apply a thought experiment,
take three dummies and repetitively heal them with T2 and T2.5 sets with a delta of 200 healing power between both for a duration. At the end you'll find that the coefficient of T2.5 (X/2.1)(H-200) versus (X/2.5)(H) yields T2 to be the top.
My point is, in a controlled environment where you setup your raid such that everyone is clumped up with a minimum amount of healers you will come out better than if you were to replace your set with T2.5 in the same scenario. In any given scenario where your uptime is less than 100%, and you have overheals 2.5 will prevail,,, as shown by your data in practice. The theoretical values are achievable, just through coordination.