There is still a second of lag when using your slaves to focus your main. I used to use the /assist method when I first started boxing, but for the life of me I can't remember if the lag was worse than the lag associated with the focus method. I've busted my EM/NS/CL macro on split targets plenty of times when I first started playing arena's using the focus method; 3 hitting the old target with my lead hitting the new target. I'm just more used to the timing now, but I'm sure i've lost matches due to the time it takes my alts to catch up with my main's target.

Before I started boxing I used to always be the group/raid leader and I had a macro to mark the kill target with a skull. When I started boxing that was my assist macro on all my alts. Basically i was already in the habbit of marking the kill target, so when I started boxing I would use a button to set the kill target (via assit) on all my alts so they would pew pew and not automatically attack what my warrior was attacking. It worked great in PVE, but for PVP that's not really what you want to be doing using 4 shamans.

I think using macros on all your characters,

/assist [target=toon1,nodead,exists,help] [target=toon2,nodead,exists,help] [etc..]
/cast spell

is a great way to start, but my concern for this is the impact of when your Toon1 [ie your main] gets crowd controlled. I know for a fact that polymorph completely screws up the targeting of my alts when i use the focus main method, and I have to switch main's (via focus change) on the fly. Sap is less of a problem because I can still control the alts via the main's window when he's sapped. Sap doesn't seem to screw up the targeting like poly does. I've had mixed results with fear in PVE (and i swear in pvp but pvp is more chaotic so i'm not positive.) Sometimes after getting feared my alts won't attack what my main (using the focus method) has targeted until I retarget. It's really a PITA but it happens so rarely i'm not sure what the cause is, however it seems to happen after fear.