I have spoken to someone that had just had his car mapped with a rear mounted(tailpipe area), it was pug 205 turbo, I asked about lag and any other problems.
No lag at all really he said, certainly less laggy than the RST he had before it, both were same size turbo's but can't remember which size, prior to this it was a conventional front mounted turbo, and he said he was really hard pushed to notice a difference in lag terms.
He didn't use a inter cooler and ran from memory 26 psi, with a alloy boost pipe rear to front providing enough cooling effect.
tabetha