I'm afraid that speed is a tad optimistic
I meant that the i2c bus clock speed would be 400KHz, not 400MHz, sorry!
Actually, you could probably avoid one of the read operations most of the time. By making the assumption that an interrupt will most likely come from the same chip that generated the last interrupt, you could read the same chip again. If the data has changed, process it. If not, then a different chip must have generated this interrupt, so read the interrupt handler chip and update the address multiplexer chip, then read the data from the chip that generated the interrupt.