My turbos are low mounted (not rear mounted more like mid-mounted) and I had lag. I had to redesign the intercooler piping for better response. It worked out pretty nice. T28's off an R33 GTR. Originally they didn't really start spooling until about 3500rpm but with the X-pipe it starts at 2800rpm.
I know a guy running the rear mount on a V6 and there is a bit of lag.