i do not believe the speed of the target switch matters if it is targettarget or focustarget. either way, the new target info needs to go up to the server, turn around, and go back down into each of your alts before being usable. (think 2 * your latency)

as for which is better... i dunno. i don't have any issues with pipswapping and using the same pip swap keys to set the new focus.

as for cc, set your '/target focustarget' or '/assist [target=focustarget,exists]' macro to a round-robin key and proceed to get new targets... hit key... get new target, repeat. this should allow you to sheep targets while continuing to caster-dps focustargets