Nothing has lag in GT5. Every turbo car in GT5 acts like it's NA.
In GT5 this is what happens:
Turbine responds instantly to throttle and achieves max speed/boost for a given RPM instantly. To get around this, all you need to do it place transmission gears close enough so that you're basically always in the powerband. Once you do this the "lag" (it's not lag) disappears.
In reality, when you floored the pedal the turbo would lag in its response, and it would be especially noticeable around peak power. So the blue throttle bar displayed in GT5 would instantly fill up when you hit the gas, but the boost gauge would take a second or two to max out. That is turbo lag.
Summary,
GT5: Turbo reacts as quickly as the blue throttle bar, there is no turbo lag, there is only the power band. Powerband issues can be fixed with transmission. Boost gauge means nothing
Real life: Turbo lags behind throttle/engine, there is turbo lag, and playing with the transmission won't fix it. Boost gauge provides valuable performance information.
There is no turbo lag in this video - watch the boost gauge
http://www.youtube.com/watch?v=jjK7ModO8Lc
http://www.youtube.com/watch?v=cOSF0LfXBlM
No lag, coming out of corners after releasing the throttle is trivial. The boost gauge jump is tied to RPM which is wrong.