For Warrior threat have you tried repeated shouting with improved shout talents while in Defensive stance & with maximum Defiance talents?
Shouting was way overpowered at launch and was nerfed considerably but even after the nerfs I remember seeing that it was still working (just had to shout more than before). I remember that, depending on the mob count, you used one kind of shout for smaller groups and the other shout for larger groups.
I think this is the discussion where in a 5man party with no pets that the change should be at 9+ mobs:
https://www.dual-boxing.com/threads/...l=1#post423140
If I read that correctly, assuming you are running in Defensive stance with max Defiance (in 5man team with no pets), I think the recommendation was 8 or less enemies spam Battle Shout and with 9+ enemies spam Demoralizing Shout. If you have one or more pets in the party the break-point for the number of mobs goes to 10 or more as the pet(s) also get buffed from Battle Shout (there is a formula in that post)...
When I say spamming I think in some cases they were recommending spamming shouts up to 7+ times before going all out with Mage damage... The number of shouts you need will depend upon your party makeup and gear so you will need to try various numbers of shouts to see what is consistent for you currently (and it may change over time as your damage output increases).
Some other threads with related discussions you may find helpful:
https://www.dual-boxing.com/threads/...l=1#post425097
https://www.dual-boxing.com/threads/...l=1#post423943
https://www.dual-boxing.com/threads/56137-SM-Armory-1-Hour-5-runs-(4x-speed)-32-GOLD-HOUR-42k-XP-(lvl-41)?p=423048&viewfull=1#post423048
You may also want to look to get some reflection damage gear on your Warrior Tank. The following tab is one that was put together more for Paladin AoE tanking but many of the gear choices can also help Warrior Tanks with aggro:
https://docs.google.com/spreadsheets...#gid=296552331
Please try specing for max Defiance and spamming shouts while in Defensive stance and report back your results. Please try with and without Improved shout talents to see if you denote a difference (I see conflicting info on that).
Priests are the quintessential healers but I agree a Holy Paladin can really help a Warrior keep aggro (I think Apathiest was one of the original strong advocates of Paladin Healers for Warrior Tanks).
Putting Salvation on the Mages (-30% Aggro) plus Retribution Aura and Sanctuary on the Warrior can really help with the Warrior's relative aggro.
Swapping a Priest to a Paladin Healer may be harder on some teams but should be fine for a WMMM+Healer team. It sucks to lose AoE heals but if you have good aggro control and aren't doing heavy self-damaging abilities (like HellFire on multiple Locks) it is less of an issue. It also sucks to not have liberal use of the shield spell but you generally don't want to shield a Warrior Tank anyway as it reduces the Warrior's rage and if you have better aggro control the shield is less needed on the Mages... Note that in an emergency Paladins can shield the Mages even better than a Priest can but need to save that for emergencies (they can only do it at most once every 3 minutes). If for some reason a given Mage does take aggro (and all Warrior Taunts are down) the Paladin healer can do Blessing of Protection on the Mage to "Shield" him...
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