Yeah, a turbocharged car is different in that it has the added factor of taking into account the boost profile.

There is no turbo boost without lag. Centrifugal compressors increase their output at the square of their shaft speed. Therefore, there is more lag at lower rpm than at higher rpm, so the easy way to make lag less of a factor is to avoid the lower rpm range when racing the car. Also, throttle response is not significantly affected by the turbo, so the engine will change rpm at about the same rate as it would if it were not boosted. The least amount of lag is accomplished by removing the turbo and running the engine NA.

In a high-horsepower application, turbo lag isn't such a bad thing because there would be traction problems if all the power were applied instantly.

The turbine A/R is relative to the turbo, so you can't really use the A/R to compare different turbos to each other. Given the same turbo, a smaller turbine A/R will initially spool the turbo faster, while a larger turbine A/R will spool the turbo better at higher rpm.