I think a important question is how much boost do you plan to run?
You are correct, that is an important question, but not the first one I ask.
To me the most important question of internal vs. external wastegate is how is the car going to be used?
If the car is set up and used for competition and racing, the external gate is the hands-down best method, as you need to be able to make last minute, or last second, change in boost depending on weather, track conditions, who is in the lane next to you, and other variables.
To go along with this precise boost control you need an electronic unit, CO-2 and solenoids to dial up or down at 1 psi intervals to run your number.
As far as a street car, my customers which "play" on the street are always in the kill mode with boost and it is limited by traction, so an internal gate is more than adequate in most of those situations.
There are various simple manual boost controllers and we are now experimenting with Turbosmart internal gate with solenoids and electronic controller mounted on the dash. This is a very affordable unit, but will provide also enough precise boost control for street application, and hopefully at the track also.
If you cannot get accurate boost control on the street with an internal wastegate, the car is not set up properly.
Some of my local GN racer friends have won season championships with an internal gate set up!
