I would let it shift automatically on a "drag race" type run from zero. The factory tune programs the shifts early, then the accumulator has to fill with fluid, then the shift happens. This is the delay you are seeing when you command the upshift manually. No way around it.
Locking it in 2nd is the only reliable (performance wise) thing you can do. This is perfect for your mountain driving as you can go as high as 75-80, or as low as 0, though 20-25 is as low as it will still pull decently in 2nd. Just leave it in 2nd for spirited driving in the mountains, as I think you already do.
If you go from drive to 2nd at speed, be sure to unlock OD so it will downshift one gear at a time.
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