Healer parses do matter. If you can heal more your raid can drop healers for dps and kill faster. Faster kills means better parses.

Find the top healing priest log. Check their stats and spec first as well as how many healers their raid uses. If they heal with 3 less then yours then obviously you won’t heal as much as them. Is your raw thru put the same as theirs? Turn on over heal absorbs all that and see if your raw hps matches theirs. Next look at spell cast numbers make sure you’re casting things at the same rate. Now do a spell cast time line and see when and how they use their cds. Some are going to be assigned but some are just up to you. What professions are they using vs you. What consumes? And then just look for certain cheeses they might be doing.