There are a few different types of VG30 turbos.
Which one is it ?
I believe they come out in a smaller version (Twin turbo Z32 version)
Big version (Vg30 single turbo) which is the turbo you want to buy
Big version with steel rear wheel (You have to be very lucky to score one of these)
If you get the first turbo i mentioned it will probably be smaller then the rb25 and only offer you less gains in power and effieciency.
If you get the second one then it will be a little better. These turbos are BB so lag is not an issue. (However you will be limited to the 14psi limit)
If you get the third turbo then your one lucker motha.... Grab it run 16psi and it will go much better then a RB25 turbo. You will get a little more lag but will probly not even notice it with this turbo. Dont hold your breath trying to find one of these because you just cant
Send me a PM and let me know what you go for once you work it all out..
L8r