Yes, it's 20% across the board. And if the host is shared with additional instances, then priority for disk usage needs to go to AirWave as it's very disk intensive. Disk contention has caused performance issues for many shared host setups (read as: don't put another database intensive application on the same host). I also suggest that the VM layer only makes sense if snapshots are in use, otherwise dedicated hardware is the way to go w/o the VM layer.
Yes, the suggestion is to go with the Pro appliance for anything 1500 devices or less. If you need to cut down the sizing, you can work with the sales team to find the right fit. But from engineering, we suggest Pro since it goes through the gauntlet of our extensive scale testing. The smallest VM setup I run is 4 core, 8 GB RAM, 80 GB disk, but it's usually 100 devices or less and doesn't live longer than a month at a time.