Practical AGL Limit  

Not sure why you need any "apparent speed over ground".  I'll fly as high as deemed necessary for the conditions.  Sometimes there is less headwind super high, or even a switch to tailwind.  You'll always get better fuel economy as you go up.  More time/distance/choices if the engine takes a dump.  I guess it could be argued that a fire in flight would be worse, but that's about all I might worry more about.  I'll cruise 8000' AGL all day if there is a reason for it.