replacing just the filter with a high flow will give you 4-5hp
Replacing the box with MS seems to give you another 3-4hp.
That is why most go with just a filter as even the 3-4 with the MS airbox seems to be hit or miss.
THe stock box is so well designed. Probably the best design I have ever seen on any car including ferraris, lambo, aston and road race cars