White Bros E-Series, XR cam & airbox mods, what to expect????

I would like to buy the White Bros E-Series slip-on, XR cam, drill holes in the airbox and re-jet. Anyone have any experience with these mods and could tell me what kind of power increase to expect. Thanks in advance to any help.

[roostfrom250ex]

The ole XR cam thing is an indacation, but what machine are you talking about?