That's simply not true at all. Look at logs of priests running 4P. Not only is the 4P heal mostly shit (there are a few niche fights where it doesn't always suck), it goes on the wrong people 50% of the time. I was actually just looking at socrethar logs of a priest I know that runs 4P and his 4P did 43% of the healing as his penance during gift of the man'ari phase. The best part about that 43% is that only 60% of that went to people who were in any risk of dying (the gift targets). That's a 25% increase on one fight to useful people. Which was 2% of his total healing for that phase. Are you really trying to tell me a 2% increase (from 80k to 81.6k hps) whereas 2P t17 is 6 GCDs to cast shields per minute (80k to 88.5k hps). There's a clear winner here and it's not the 4P.2/2 is worse than 4pc T18 on the majority of encounters that don't feature strong healing required on singular targets, basically only limited to 2 encounters this tier where it's not a waste of time to fully channel a 4 tick def Penance. If you can't fully channel 3 ticks consistently without overhealing, why would you suddenly do more effective healing with an extra second to channel?
4pc not only reduces the probability of overhealing with Penance caused by 2pc T18, but increases your overall output as well. The 4pc bonus targets those with the least % of health, so it's always going to go to someone who needs healing at some point, even if it's not necessarily now, and thus isn't the situation you described where the proc doesn't go to someone who needs it.
In any case, there's just no good reason to run 2/2 with the throughput and base stat loss.
And a few things you got horribly wrong. You don't use t17 2p for the extra channel you use it for the stack. You can channel it for 1 tick 2 ticks or 3 ticks, hell even 4 ticks if you really wanted to but you can stop it at any time it'd overheal and stopping it at 3 ticks is no different than casting a normal penance. And because heroic tier is only 710, it is certainly worth it to skip tier till mythic 725/730 mythic gear. 1.25 lower average ilvl won't have a lasting impact on your throughput that extra GCDs can't make up for.
How the hell do you figure 4p makes your 2p overheal less. They're completely independent, if you're 2P is going to overheal then why are you casting it to begin with. If it's ticked twice you can stop casting and get a shield out, if it ticked once then ya I could see your point but you'd still stop after 2 ticks. So you have a 50% chance to generate another penance heal that could also very well be useless.
Do you not understand that healing someone with the extra penance who can be healed to full by passive hots, healing AoEs is useless in 9 out of 10 scenarios? If you're on socrethar and the penance doesn't tick to a person with gift then it was 100% useless because no one is at any risk of dying besides those 4 people. If someone was at the risk of dying, they'd probably end up dying fairly quickly and leaving it up to RNG to heal them is absurd. Same for kilrogg, your heal jumps to someone got low from death throes but because you shielded one of the people with heartseeker they don't get the penance jump. Aside from the end of kilrogg, no one is going to die to death throes aoe so that heal was useless. The same can be said for atonement heals. Yes mathematically they seem fine but at the same time you could very well heal someone who doesn't need it. Looking at socrethar the only reason people with gift got healed by atonement or penance jumps was cause literally no one else was taking damage. Any time AoE went out or a monk/dk tank dip below the threshold (they're at 50% while people with gift are at 60%) the heal is wasted. Sure a 100% heal is better than nothing but I'd rather have a GCD than that heal.
As it stands, I plan on using 2P T18 + 3 mythic offpieces for more than 60% of the fights in HFC. The only time I'll run 4pc is for gorefiend, HHC, Tyrant, Fel Lord and it will be 100% to parse at that point. Parsing highly doesn't mean you did what's best for your guild.
If you don't believe that this is better throughput (ignoring talent choices and play) take a look at malicrine a disc priest on US-KJ. He's consistently parsing top on his progression kills and he runs 2P/2P.
4P heal is a useless padding heal that is unnecessary for progression.
But I don't know why this thread has turned into this so I'll just end this here because this is not the right thread for this kind of discussion