Hit% is a fundamental concept of how to calculate average damage (It's also part of the feat description). And you either are missing it (charitable interpretation), or are just ignoring it (the least charitable interpretation). It wasn't meant to be an insult. Not sure how else I could have worded it.
To be fair, it depends upon how you define damage. Damage per attack, or damage per hit. We often look at DPR in terms of damage per attack, but the problem with that is that the chance to hit will have a lot of impacting variables that change considerably between games, character situations, etc... If a campaign uses the alternate 'flank = advantage' rules, advantage on most attacks can be assumed. If you don't, advantage may be rare. Also, the typical AC (and hp total) of enemies has to be factored into the equation - if you're fighting though an army of AC 18 hobgoblins, AC 15 trolls or AC 13 Orcs makes a huge difference. After all, you can't deal 30 damage to an enemy with only 7 hps.
As an example, I run a dwarven barbarian 14 / fighter 4 in one game. He consistently attacks at least 4 times per round and deals at least 25 damage per hit. He had a run of over 70 attacks in a row where he did not miss, even though he has NEVER attacked without using Great Weapon Master for -5/+10 (if it was possible to attack with it). There are just too many factors making sure he hits (reckless attacks, bard bonuses, rerolls, etc... ) For him, that feat is essentially 10 free damage per strike. Another PC, a fighter I played up to 9th level, had GWM from 1st level on, but often did not use it because enemies in that campaign were just too high in the AC department (the majority of the foes we faced were4 heavily armored). The impacts of that feat on damage for both of those characters was drastically different.
Regardless, Tempest Clerics usually don't get Haste attacks, often do not get reaction attacks (investing in mechanics that compete with the reaction ability you get at level 1 is not something many priests do), and only rarely get bonus action attacks - meaning that they generally get GWM damage bonuses on only the one strike they do. As they have an 8th level mechanic that bumps damage up for them, but only if they hit, the cost of using GWM is relatively higher than for a fighter without that bonus damage. Accordingly, although it is not a no-brainer, Tempest Clerics may find GWM to be an unwise investment.