1. Starting the analysis in 1975 is misleading (the Ford administration). The most interesting cars are from the late 60s early 70s before emission regulations, oil crisis, car bumpers, ect kicked in (and dealt a huge drop in power, increase in weight, ect).
2. Cherry picking a muscle car for its weight is also misleading. Muscle cars weren't light. Some had 6 or 7 liter engines (That's a lot of iron!). They were quick-ish because they were powerful. Muscle cars might be the only cars to have lost weight in the last 40 years, since its ridiculous to have a "sports" car weigh as much as a family SUV.
Look at the accord, or the civic and see the weight gains.
In fact, cars have been getting lighter but over the past 5 years, because they've become ridiculously heavy. This is new and excited (they're still whales though)
3. Yes my SUV has 300 hp, which is insane. But I go around a mountain corner much slower in it with it's 4000 lb than I would a FIAT 127 (kids, remember to downshift to keep the drum breaks cool!).
Point is, a car isn't a good performance just because it has a good weight/power. Just the weight itself is important (light always wins for the same power weight ratio).