Turbonetics GT-K turbos are nothing special. They are comprised of Borg Warner cartridges. Borg Warner turbos do not last long, however, they are becoming increasingly popular. They are intended for Truck use (mostly Semi). Very good bang for the buck....but not the GT-K series. Overpriced.
Turbonetics USED to use Garrett cartridges, but Garrett dropped them a couple years ago. Thats why they are using Borg Warner cartridges now.

Garrett BB technology is the best.

The main reason why they spool faster is because they reduced bearing friction. Basically, they have MUCH less moving parts, therefore, making the cartridge lighter. This allows for much faster spool time.
Also, they can handle higher boost pressure (partly in fact because they are oil and water cooled).