Originally Posted by BMRLVR
Not a lot of headroom is required to achieve full flow characteristics of the stacks due to the nice smooth bell mouth opening that funnels and directs the airflow.
I'm no expert, but it seems to me like there is very limited airflow to work with under the hood. Air isn't actively being pushed into the stacks and the engine has to work harder to get the air it needs.
Kinda just talking out of my ass but the above seems logical to me.