What is this turbo good for?

Interesting point.....Think of the exhaust pressure and velocity; the net airflow in will be the same regardless (no throttle plate as you noted), but when you add fuel and ignite it, the exhaust obviously gets hot, expands, and has higher pressure and velocity. And, looking at it another way, with a hotter and hotter exhaust as more fuel is added and burned, there is more energy content in the exhasut flow to work with.

Kinda works the same with cams and turbos on gas engines. Long durations and overlaps are a no-no as it drops the velocity and pressure of the exhaust/exhaust pulses.

I suspect spool speed in this case has to do with turbine mass as much as anything. It is pretty darned large compared to the 16G Super I use on the Starion. And, the 16G Super has the the impeller fins customized for the quick spool up; dunno about this RAM turbo.