If you look at a performance chart, you'll see that the highest true
airspeed for a normally aspirated aircraft is more likely to be around
6000 feet or so.

Isn't that for something less than full power, i.e. 75%? I'm talking max
power.

Here's a c'n'p from this page:
http://www.nar-associates.com/techni.../altpart1.html

"The right hand intersection of the curves gives the maximum velocity in
level flight at that altitude. First, notice that for a normally aspirated
piston engine the maximum velocity in level flight occurs at sea level."