Everything gets countered heavily due to AoE when playing amounts of 10, simply due to AoE damage not splitting.
Druids are still above the rest for their utility imo. Round robbin typhoons will keep enemies at bay whenever you want, you can shapeshift slow on whole team, you have stealth.... Sure you may neglect a fear with a shaman or a dk team but it doesn't make them superior, when you peel properly the druids will always live longer.
I'm sure you know what I mean, you wrote it yourself => every team you tested equals the druid team.
What budget do you have for the PC?
You can't compare it to chain lightning, Mindsear is aoe while chain lightning isn't.
About the damage though, it is very strong especially when stacking haste.
Drive your priest team with a warlock main (take spelltank demo-destro hybridspec => the warlock will not die easily to other boxers or any type of spelldamage) and you won't need healers. Divisiona ran warlock + 9 priests and he kicked *** with it. When you get casts off you will flop boxers pretty easily. Vs premades or a bunch of solo players it is a different thing tho, you will have to split your mindsears in order to do effective AoE damage. (Or wipe them 1by1 but it will not end well for you if the opponents know their thing.)
This combo gets strong really fast when upping the amounts and when you have access to haste gear.
Main bottleneck to priests is mana, even with dispersion and hymn you'll face scenarios where you will oom and die.
If you need more info feel free to ask, I have a 40 man team with 32 spriests in it on Icecrown.