I am still researching my question about the benefits of sourcing the air from ambient and I found this discussion in the link below. It sounds like if you have the ST200 airbox with a hole on the bottom then the added pressure from the ram air big mouth at speed is just going to get pushed out the ST200 hole. Maybe not as much though if you have the ST200 airbox inlet routed to the front of the car which would be at the same pressure as the big mouth.
http://www.fiestastforum.com/forum/threads/7210-Fiesta-ST-Ram-Air-BIG-MOUTH-Intake-Snorkel-by-Velossa-Tech-Engineering-R-amp-D-thread!/page15
http://www.fiestastforum.com/forum/threads/7210-Fiesta-ST-Ram-Air-BIG-MOUTH-Intake-Snorkel-by-Velossa-Tech-Engineering-R-amp-D-thread!/page15