RE: How High?
If that 400 foot ruling applies to all areas and not just around airports it means that any sort of sailplane flying with thermals involved is going to pretty much require an application for a special exemption and a resulting NOTAM to cover the soaring field and surrounding area. And that assumes that the FAA would be willing to issue such permission.
It would be pretty much impossible to carry on any RC soaring or serious free flight contests with a 400 foot restriction. Heck, even a turbine meet would be "tight and tough" flying with a 400 foot restriction.