"They'll beat anything Shelby designed every time" Shelby's Cobra ate up many a Ferarri, Porsche, etc..., in its day; as did Ford's GT40(after receiving help from Shelby). And, he did so on European turf.
The Europeans have nothing on American autos today. Shelby's Series One, Panoz's Esperante & GTR, Dodge's Viper, and Ford's new GT40 (among others) will hold their own against any European challenger.
Incidently, Jaguar & Aston Martin are both Ford products.
And Lotus is owned by Proton (Malaysia).
A lot of what you're talking is horses for courses. Put that 230hp Caterham or a 190hp Lotus Exige on an oval and theyll get killed against any of the better American sports cars.
However, put the same on a tight, twisty track requiring excellent handling and road feedback, and the American cars will be left in the dust.
I have a video of a 200hp 1.8l Elise keeping up with a 400hp+ 5.8l Corvette at Nuremberg ring, except it fell behind on the long straightaways. The driver almost passed him once on a curve, but he screwed up his line. The Corvette also had a big head start.