To put it into numbers in case it's still not clear

If you do BroilII -> Energy Drain -> BroilII you do 230+150+230 potency over 5.75s (2.5+0.75+2.5, even tho ED is instant you have that animation lock Grimoire-M just talked about, and you have to take it into account). So we have 610potency over 5.75, so 106potency per sec for these 2 gcd

If you do MiasmaII -> Energy Drain -> BroilII you do 200 + 150 + 230 over 5.00s (here you don't take the 0.75 because it happens while the GCD is ticking because MiasmaII is instant)
So we have 580 potency over 5s, which is 116 potency per sec

To do a proper dps comparison you would have to do it over the course of an entire 45s cycle. But what I just wrote should be enough to show you that it is in fact a dps gain to use Miasma2 to weave ED.

Altough it is a dps gain, I wouldn't call it a "major dps gain"

Clipping all 3 ED (or any ogcd actually) would make you loose 2.1s of dps which is almost a broil2. While they will effectively stack up over the course of a fight (assuming you clip every single ED) that would result in a loss of 6-8 broil over the course of a regular fight. That alone shouldn't break your dps. In my personal experience, most healer loose much more just by not having their GCD rolling all the time which is the first source of dps loss.

However, SCH having many ogcd, if you clip absolutely every single ogcd, not only the Aetherflux spells, then you will see a major difference in your dps.

This issue could be easily fixed if SE would simply apply AST treatment with Malefic to the two other healers