The speed limit is totally irrelevant to this discussion, other than me making the point for effect that there are numerous speed limits in the USA that are 80mph and one in Texas that is 85mph. The question is how fast people
actually drive. If most people drive 80mph, it stands to reason performance in the 80mph region matters for a car. Are you really asserting that the average speed of traffic on major interstates in normal conditions is 65mph? I don't think anyone would be disingenuous enough to assert that since it's obviously not true to anyone who drives - it's much higher. Fact: the vast majority of drivers speed - significantly so - and any argument about "who needs to drive 80mph since the speed limit is 65mph" is frankly ridiculous in light of the fact that the national average freeway speed by drivers is higher than that.
Here is one study that showed the average normal driving sped was 66mph on interstates with a 55mph speeed limit, 74mph on interstates posted at 65mph, and 78mph on interstates that are posted at 70mph. These averages
include crowded coastal interstates and take into account the speed on average, so in reality the speeds on sparsely populated stretches will be significantly higher on average.
Fact: If you are going 80mph in many, many areas of the country, not only will you be getting consistently passed by faster traffic - you're going slow enough where the cops won't look twice at you.