I've gotten mine up to 94Mph, but I let off when I got that fast because I knew that it would take a lot longer to get much faster, and I didn't want to get pulled over... no way to explain that you were just seeing how fast the car could go...
Wouldn't engine temperature also give you a good idea of how well the engine and all its system run at their best? That 45-50 mph 2000-2500 RPM range keeps the best engine temp (right over 80C) with no fluxuation like I get when I run at 70+ on the highway, it will get closer to 100C on the highway and even a bit higher with the AC on. At 45-50 mph the engine temp doesn't move w/ AC on or off.
I dunno though.