Exhaust Length

[twitchy]

I was told that making your exhaust system overall length longer will allow your engine to generate some extra horsepower.

For example, look at the BMW M series, coupe and roadster. The roadster has something like 15 horsepower less than the coupe since the exhaust system cant be made long enough in comparison. This leads me to my 2 questions:

-how does removing your exhaust system make sense in this case (a la race car, etc)

and

-if I was to make my exhaust system longer when I redo it in the spring (make it "zig zag" underneath) would there be any benefit?