I suspect that a big limitation in shift speed is often the angular mass of the crankshaft and flywheel - that energy has to go somewhere in an upshift, and doing that quickly on a typical car with wide gearing and a heavy flywheel puts an incredible amount of stress on things. Gear ratios cancel out in the real world, since with close ratios you're also shifting more often.

I would expect something like the GT-R to have much less crank/flywheel angular mass than the average car, as well as beefed up transmission components. Even so, IIRC they had to dial things back a bit because transmissions were blowing up.

DCTs get rid of the delay from matching the input and output shafts of a conventional manual. Very helpful, but not a cure-all. My car (automated manual, non-DCT) spends much more time matching the engine to the input shaft than the input to the output shaft.