Lag has to do with turbine sizing compared to the engine...
on a rear mount turbo set-up... you just have to size it differently than you would on a conventional set-up. Then you don't have as big of Lag problem as would be expected.
However I would imagine the rear mount is not as efficient as it "could" be...the losing of heat mostly...but it does work.
Easier to install and uninstall...they get pretty good gains.
