Thanks for your input.

In my dps macros I have

/Assist Toon1
/cast whatever

So whatever toon1 is targetting all the other toons dps like hell.

This method wont enable me to silence a caster mob with one toon while the rest is targeting/dps another mob.

Regarding the healing, I don't think I have ever healed a wrong Toon due to lag, and for me its fine with defining a specific key to heal a specific toon, instead of using mod:shift+alt+1+2+3 to heal toon 1 etc. as I have seen some players use.

I might try using focus a bit more, but as I said, in the beginning I had trouble gettinng it to work properly.