A 5:1 ratio (20%) for the cowl is too long for maximum drag reduction. A fineness ratio of 3.33:1 (30%) has less drag.

Control line speed uses line size to control speed. When engine and airplane improvements reach the speed trigger point, the line size is increased in the next rule cycle. It's been done this way for decades.

Cooling drag on full scale aircraft is significant, generally 10% or more. The theory is to slow the air down inside the cowl (which is done with expansion) so the air has a chance to gain a greater delta T. Then the heated air needs to be accelerated to the free air stream with as little turbulance as possible.