I don't see why it would leak unless you buy an intercooler which leaks to begin with. No you will not lose boost pressure, unless you buy a said leaky intercooler. /ubbthreads/images/graemlins/banghead.gif
The inlet and outlet of the Evo3 intercooler are not in the same spot as the VR-4 intercooler, so you'll need to re-do your piping as well as make new mounts.