Originally Posted by Mike Rainbird

The Cosworth airbox was designed to suit power increases for homologation purposes, so is very good up to a it's air flow limit. Beyond this (around 500bhp) it becomes restrictive.

Modern cars have different constraints (emmission compliancy / noise compliancy / safety compliancy (dictating the amount of room available) / cost compliancy (using generic items from the parts bin to keep costs down), so these systems can be compromised and very restrictive from the factory (the Focus RS / ST are a prime examples).