Im with Roy. Before spending say $5-7k+ to do a conversion maybe look at a better set up on what you have. Im guessing if you have a problem with lag you got a bigger turbo, so maybe manifold design, after market computer, some head work, cams etc. And you would have spent a fair bit less money.
And end of the day, rb20 goes boom, its not as much to get a new motor as if a rb25 goes boom
Just my thoughts anyway.