Easily American cars. I could build a mustang that runs high 10s for under $4000. Including the price of the car. No way you can do that with an import.
You could go to a Ford dealer and buy a new mustang GT, get it retuned and slap some drag radials on it and run 11s, nothing else needed.
You also aren't running 11's with "just putting on a turbo and a new intake". To get even an STi into the 11s requires several thousands of dollars, and if you're willing to spend the money, can be done on a stock turbo. Flash, fuel system, intake/exhaust/pulleys/cams/tires and you can get about 12.40-12.50 from an STi, and all that will cost about $3500. Another $500 and I could have had a mustang running 10.90s.
And I forgot...You could go to a Ford dealer and buy a new mustang GT, get it retuned and slap some drag radials on it and run 11s, nothing else needed.
Also, if you see muscle cars running 11's, they are far from having "just about every performance part done". You see, unlike the import world where there's 1 aftermarket intake, and you can only use your stock cylinder head(s), there are a variety of options for American V8s. Take the Ford Mustang for an example. If it has a 302, a 351W will bolt right in. You can make a 302 into a 306ci, 331ci, 347ci. You can get the 351W, or you could turn it into a 358ci, 373ci 393ci, 408ci. All the heads interchange on those two engines. There are approximately 40 different performance cylinder heads available from a mild, stock replacement iron head that would be worth about 30hp gains over the stock heads, all the way up to huge race aluminum heads made to flow enough air for a big supercharger or huge turbo on that 408ci engine. And then you can port those heads further.
There isn't just "stage 1, stage 2, and stage 3" cams to choose from. There is several HUNDRED grinds of camshafts for a smallblock ford V8. There are about 50 different carbureted intake manifolds and about 30 EFI manifolds. You can use a timing chain, timing gears or timing belt. You can even change the rocker arm ratios to increase lift without increasing duration of the camshaft. There's multiple manufacturers of aftermarket blocks. If I wanted to double my budget, I could take that same mustang, and for about $8000 (not counting safety equipment) build that mustang, including the car, to run high 9s, with a 351W based engine, with no turbo, no supercharger, no nitrous, and on pump gas, while still remaining street legal. There isn't an import ever that can do this.
Lastly, most of the muscle cars you see running 11s are still using pump gas, mild ported stock heads or mild box stock aftermarket heads, still run the stock suspension, and only have upgraded camshaft, intake manifold, carburetor and exhaust, in 3800lb+ all steel bodies.