Disc still came out on top for BQL, but slightly below on Sindragosa. But Sindragosa is a special case, has long periods of time where you can't actually cast (HM), with the raid dipping quite low in HP, cutting HoT overheal down 30-40%. Plus the fact of the timing of sequential Unstable Magics, and the timing of Ice tombs (tomb in phase 3 = 10-15sec down time... when the raid is in the most need of healing). But, I'd probably never go Holy for BQL, but I do switch to that for Sindragosa when we have 2 paladin's in the raid.
You're linking to a kill where you have too much healers, considering the overheal you have.
It was being said here that this is typically the case where Disc shines on meters, because they are better to snip heals.
But shining on meters doesn't mean shining in death prevention. You're setting here at 10k max hps and 70% efficiency, where all other raid healers are at 15k and 40% efficiency. If you go with one healer less, your own effective throughput will not increase a lot, but holy priests (and shamans and druids) will be able to go up to double throughput. I can have less than 25% overheal on BQL, and holy priest have been seen over 10k effective Hps (which is more than your total hps, counting overheal).
In other words, consider the following events. A player a full health is hit by an aura ticking at 3k each 3s. He gets a shield of 12k, and a hot ticking also at 3k every 3s (for 12s). The game system will consider that the shield is fully effective and the hot fully overheal. However, from death point perspective, both would have negated the damage on their own. I agree that preventing damage is better from a death prevention point of view (due to the risk of death damage : you can't heal dead people), but the difference between 12k shield and 12k hot is not that big !
Part of the hot overheal comes from shields being applied on hotted targets.
[regarding Festergut] :
The main question is where your trouble is. If you loose people due to Pungent Blight, by all means, go with pre-shielding (and use correctly raid bubble wall / personnal cd / stance switches / etc.) . That's the most effective way to deal with it. However, if you struggle to keep your tank alive with 3 inhale stacks, then the extra health-buffer and fast heals that disc can provide can be also a huge difference. Once you have 2 paladins, you're not lacking sheer throughput to keep your tank. You're lacking effective health, and disc'tools is a lot better than holy slow great heal or little flash heal to face that problem.
I acknowledge however that healing the tank will hurt your meters, compared to pre-shielding the raid.