The modern Hemi engine uses a manifold that is a close copy of a shorty header. And it will outflow an old log style manifold by a lot. And every header manufacturer still sells a bunch of headers for them and have proven gains. Any header design that offers a performance increase will help make an engine more efficient, which gives more power AND fuel economy. Maybe adding 10 horsepower on a low performance engine that only puts out 230 horsepower doesn't mean much to some people, but increased efficiency should .
Keith