Ugh, rear mounts are so laggy. Really only something that you want on a dedicated race car that you can stage and build the boost. TONS of intake volume to fill. A friend had a 4th gen WS6 with a rear-mount setup, made 550 whp at peak, but it took so long to come on that on the road course my 350 whp 1993 RX7 (factory 2 rotor wankel, factory turbos with increased boost pressure and supporting modifications built for SCCA Solo II SM class) was able to pull away enough just by being able to get on it earlier that he never could close the gap on the long straights, and I had so much power coming out of the corner and he had to wait so long I kept getting further and further in front of him. He’d freight train me at the drag strip though.
For a street driven LS, I think the best all around setup is a centrifugal (I’m a big ProCharger fan) blower on an LS3 or new LT1. 625-650 reliable, all day horsepower. Plus blower noises and no extra heat between the VCs like you get with a roots. Meth injection can really keep those intake charges down too and net you an extra 25-50 hp depending on tuning and blend.