Bit of an old topic, but I think the turbo is very well simulated.
Turbo spins trough energy in exhaust and depends on these factors:
Amount of exhaust gas (RPM and Throtlleposition)
Temperature of exhaust gas (hotter = more energy, hot gas expands and creates more volume to spin turbo)
Load of engine (well, more torque gives more push in exhaust)
These three combined will not get very linear turbolag.
I think the lag is very good, when off-throttle, you need about half a sec to spin back the turbo. I am experiencing it very well with the XRR on Westhill. The first lefthand corner, you need to brake a tiny bit and step on it again. When you do that with one foot, you have a turbolag. When doing a left-foot brake and applying full-throttle to keep turbo spinning, you accelerate out of the corner much quicker.
This experience is very realistic to me TBH. (Oh, i drive a Renault 5 GTT, so i know turbo LAG
)
The reason you don;t get full pressure in 1st gear is that the the amount of time is too short, when hitting rev-limiter the torque drops massively, so no turbopressure there.
But, I found this topic to report a bug in the game, and don't know where to put it:
When out of fuel, applying throttle still spins the turbo (testen when rolling)
That's impossible.