I'm pretty sure I2C will be too slow. Even if you could take advantage of the 1Mhz bus timing, you state the "output PIC" will be operating in the uS time frame. I take this to mean you will be doing something every couple uS or so. It will take at least 20 or so bits per memory access (address + command + data)at 1Mhz thats 20uS. Not to mention having issues finding devices to support the 1Mhz bus speed. So that puts you at 400K, now your timing just got 60% slower.