We've tested with something like 80 Z-Wave devices and I think upwards a 100 ZigBee devices, but we've never tried to test the limit.
Z-Wave is limited to something like 250 devices.
What we do know and what we're working on improving is that there's a slight slowdown once you exceed about a 100 devices, but it still all works and we're continually making improvements to our Z-Wave and ZigBee stacks. It won't be a UI slowdown, as the processor we have is more than capable of handling it, the slowdown would be in getting the data to/from the sensors/devices.
We're going to get some kind of sorting, most likely based on the location done, as we know it'll become unmanageable quite quickly once you go beyond 10-20 sensors/devices as things are.