May I ask why you want to control speed of all cameras from a remote place, when if they are kept at P or other semi-program settings you can achieve a good result as planned before, only shooting all at the same time?
You know this is how the genius guys in Matrix first came with showing an action frozen in mid air as cine camera (Several DSLRs in circular fashion place around subject) go all around showing the action from every 360 degree angle... & did the magnificent especial effect that was unheard off before.
Not sure about Nikon but my Canon cable does give that power to shoot as many as cameras as I want by serial connection of cables between them to do the effect shown on matrix and on several other major Hollywood movies after that and is shown in good ref. image Chunshine is left for you. I am sorry for large number of Nikonphilic friends here but I have to say the same thing Chunshine said especially when you want to invest in large magnitude. You definitely need a system that has maximum expandability, capability and actually does the job for you when money is not the barrier otherwise every camera takes picture. Yes You need Canon!
(I guess Nikonphilic friends are now enough stimulated to give you some good advice so if you want to thank me, send a whispering email)
