In very simple terms, there is LESS than 1" of vacuum behind the throttle body on the GT even at the 18 psi level. This has been verified on my airflow bench as well as the dyno and the track.
The size of the inlets on the car and airbox are more than adequate to support 800 rwhp all day long. Again, I have airflow tested all of the components of the air box, with and without filters, with and without air inlet tube, as well as both sizes of throttle bodies.
And airflow did NOT change with K&N style filters.
The root cause is very simple. The durometer of the rubber hose is simply too soft and unsupported. It works fine when it is cold and that's about it.
The airbox assembly flows over 1300 cfm with stock airfilters installed. There is no cause for concern regarding airbox size or design.
Best regards,
John