The typical answer is that you "should" be able to use the restrictor pill that comes with the STi or stock VF-39 turbo. Its slightly larger then the #57 drill bit that they recommend you use if drilling out the stock pill that comes with a wrx / tr setup. That pill is very tricky...too big and you will have troubles obtaining the max boost numbers put forth in the Cobb OTS maps. Too small, and you risk a boost spike in the higher RPMS (not to be confused with boost creep!). I've put VF39's and VF34's, and sadly it can be hit or miss. That drilling can be a B@tch to get right!