I have seen adapters on blueovalchips.com for injectors not sure if there right for the inkectors you talking about. Also 460 cc might be enough for 300 on an rx7 but i don't belive it is enough for for the zetec motor. For 300+ you will probably need 60# or roughly 650cc injectors from my experience.
I don't know what the Focus uses, but they have to be the right resistance. Low resistance/impedance injectors are typically around 2.5 ohms, high are around 12. If you vary from what your car is designed to use, they either won't get enough current to work, or they'll get too much current, work for a millisecond, and then pass the excess current to the PCM and probably immediately fry it since the PCM is not designed to carry any real amperage. I'm sure Tom would have explained this if he were a more talkative guy.
You would never see any benefit from switching injectors on a NA car anyway, the PCM dictates what the injectors do and when you floor it, any stock fuel system is way more than capable of delivering the right amount of fuel to give you every bit of power the engine is capable of. Unless you have a legitimate problem in your fuel delivery system, you have nothing to gain by messing with it.