Ok so I found a reference here:http://arduino.cc/playground/Main/WireLibraryDetailedReference
It seems the hardware would be perfectly able to pull in the bytes and then notify me of the fact that data is waiting but the wire library at present ties up into waiting for transmission to end before releasing the processor so your interrupt is still called to signal available data but the MCU was tied up prior to that inside the library too. That's a bummer. The communication isn't really critical that it get there at a 2MHz clock rate but tying the processor up staring at the TWI hardware even at 400KHz seems wasteful.
Anyway, if I2C turns out to be better for me to go with then I could always send the data a couple bytes at a time with pauses in between. Even if the data took a milli-sec or two to get there it's not a big deal. I'll study up on the SPI routines shown in the other link too. Maybe even see if I can alter the wire library to not block.