I personally wouldn't determine (speed-) performance being a matter of rim size, as 15" are still more or less standard on modern mid-class-vehicles, that probably reach higher speeds than a standard average ebody.
I'm not a racer, the difference I see is the grip and handling during normal ways and conditions of driving.
And I really don't feel very comfortable going anything faster than 70-80 mph with the Challenger, and this only on a straight Autobahn without traffic. No problems with any other car though. Of course I don't forget the age difference, but if braking is extremely different on dry and wet surfaces I blame the tires for 95%. I would even bet any standard Goodyear, Dunlop, Continental, Michelin etc. would perfom better on a Challenger compared to the Cooper Cobras - but - do they suit an ebody? I guess everybody has to decide that on its own.
You are correct that there are modern vehicles that still use 15" diameters wheels. IMO, the problem with these tires is that they are only available in widths that range from 175-225. In other words, way to narrow to really provide effective grip on an E body that needs 255-305 sized tires. As such, a UHP tire in 205 still won't grip much, if any, better, than a 295 T rated tire with 300 treadwear.
While speed ratings are only part of the formula that determine a street tires potential performance, typically better performing tires will have higher speed ratings, lower tread wear ratings, and higher temperature ratings. Most muscle car sizes are lacking in at least one if not two of those categories. Race tires are a whole 'nuther story.
Stateside, the problem with Goodyear, Dunlap, Continental and Michelin is the same problem I stated above. None of them make a really good 15" diameter tire in a width that will fit nice, look good, and perform well on an E body, or any classic of the same era. All of them did at one time, but that time has passed. The market is basically filled by half a dozen companies, none of the above companies are included in this group, producing tires that look nice, but have no true performance built into them.
