You raise an important point and one I'm quite aware of (I have driven
cars that felt unsafe above 70mph, and others that felt fine at
100...but weren't).

I used to have an old Super Beetle. That was a car that I'd never drive
about 70mph (most times I couldn't get to that speed.) I used to have
an old Nissan Sentra. That car felt unsafe above 70. I used to have a
VW Fox Wagon. I drove across New Mexico at 100mph in that....until I
discovered that the brakes weren't up to slowing that car down from
that speed.

When I upgraded the engine in my Westy, I did NOT just upgrade the
engine. I also upgraded the brakes. I would not have felt comfortable
at those higher speeds with the stock brakes (especially now since I
also run larger wheels and tires).