The v2 board is faster with normal HC-SR04, cutting off the 200ms no-echo units at 50ms, and doing the same with the bad units that never come back, but that doesn't address the blind spots at the corner of the robot or the hassle of handcrafting mounting brackets.
The code does support multiple boards but that's klunky.
I just fired off prototype designs to OSHPark for a 16 port board based on the PCF8575 and a little board to mount 3 HC-SR04 on each corner - with two at right angles on one side and a third at 45 degrees on the other. The idea is to mount it on standoffs.
The extra V and G pins on the connector blocks should allow the three sensors to be run from 2 4-pin cables by inserting them at right angles.
Any interest? Comments welcome!