airbox vs air filter

[mgglep]

Any of you teccy wizards out there know which is best on a set of motorbike carbs:

An airbox with a cone filter on the front or a pipercross large dome filter

The reason i ask is a friend has a stage rally mk 2 escort with throttle bodies and a pipercross dome filter (PX600 type) he had it rolling roaded and the guy explained if he converted to an airbox and filter on the front (like a plenum chamber) he would get another 5-10 BHP

Any thoughts would be appreciated