Why is this massive flow capability from a small wheel important? Transient response is a function of rotational inertia. Anything you can do to reduce the inertia of the rotating compressor and turbine wheels will improve transient response. Remember, inertia is a function of mass x radius2. In going from the GTX Gen2 71mm compressor to the G25-660 67mm compressor wheel, the inertia reduction due to mass reduction is
~5.5%; this assumes a straight scaling of mass with diameter, which should be close enough for back of the envelope calculations. Edit: I had a bit of a brain fart. As the compressor and turbine wheels are basically cylinders (volume of a cylinder = PI * r2 * height), the mass should scale by radius2. So the mass reduction is ~11%. The inertia reduction due to decreasing the diameter/radius of the compressor wheel is ~11%. The result has a total compressor inertia reduction of ~20%-21% while maintaining the same flow capability. Again, this is a back of the envelope calculation.
Looking at the turbine side, The G25-660 uses a 54mm diameter turbine wheel in Mar-M, whereas the GTX30 uses a 60mm wheel in Inconel. Mar-M is slightly less dense than Inconel by ~1.4%, based on the datasheets I could find, so we’ll just call it a wash. The inertia reduction due to mass reduction is a big ~19%. The inertia reduction due to radius reduction is a matching ~19% due to inertia being a function of radius squared, so a radius reduction has an exponential effect on inertia. The total inertia reduction on the turbine side is ~35%. The turbine wheel is much heavier than the compressor wheel, so a greater inertia reduction on the turbine side has even more impact on transient response. It’s fair to say the G25-660 will have significantly snappier response compared to the GTX3071R Gen2.