Dunno how much this may help...as you may have already tried all these

but a few addon suggestions to help like

Jamba... and make sure team is set to follow under team ....and always strobe on master under team options follow...and hit the push settings button to all toons
monkey speed to see each toon speed max

Video on screen usage(non addon)
Riva Tuner/MSI afterburner...for Nvidia cards

My killer network app shows only 5% of my PC traffic is WoW

Can also check your router to make sure your PC is getting the highest priority and the WoW port traffic is set to best QoS quality of service...especially if there are others sharing network in house and streaming/downloading frequently while you are playing

That said crossing zone lines cause follow to fail and taking off flying from the ground and from water still cause issues with 5 toons on one PC for me...

The toons followed better when they flew slower...ie with just the first agonizing slow flight license speed

I have much better luck splitting them 3 and 2 and using 2 PCs...running them at a 3 setting generally pushing 80-90% video on main

Some people have 10 toons on 1 PC...so I've read so your mileage may vary...but I am using SLI dual video with a SSD and a killer network card meant for gaming on my main and stopped running 5 on it...just my main 3 when using mixed tank/healer/DPS and other 2 dps on laptop