Well not only turbocharged cars has nice aftermarket exhaust manifolds, these days you can see the natural aspirated cars are using the same concept for better air flow. Well a few JDM tuners did make prototype versions for track use which were slightly conservatively designed than Bisi's but those werent for mass production.
Some massive extractors do need extra clearance like removing your air-conditioning system and also relocation of your radiator. Some use thinner aluminium radiators.
I think you need good power to be able to "breathe" through these big fat extractors.
Check out Bisimoto's website in one of my links.
ps. exhaust manifold/extractors/headers....depends on which country you're from. ha ha.
