Gaining horsepower from tuning is about finding the “optimum”air to fuel ratio. The more “extra” air one adds the more fuel needs to be added. The “tuning” adds the extra fuel. Add enough air (supercharger/turbo, cylinder heads, etc) and larger injectors and a larger fuel pump is required, as well as a tune.
The further the air to fuel ratio is from “optimum,” on the baseline the higher the gains are once it’s “tuned.” Generally speaking the stock tune isn’t that far off from “optimum” and bolt-on parts don’t add enough air to where more fuel needs to be added via tuning. Once one of my cars was tuned n/a and it only gained a few horsepower via the tuning. When it was tuned after it went forced induction there was a 90hp gain from the first pull to the last. The actual performance parts added added horsepower as well, the 90 hp is what tuner was able to “tune” out of it. The baseline air to fuel ratio was terrible.
I don’t have any experience with hp tuner, but I imagine the bulk of the tuning that you’ll be doing will be playing with the timing advance, rev limiter, and shift points if you have an auto and the tuning software lets you do such things. Advancing the timing is usually good for a few horsepower here and there. You’ll probably have to play with it to figure out “the sweet spot.”
These are all just generalization statements about tuning. I don’t have any Nissan tuning experience but I’d venture to say you’ll gain as little as five horsepower or as much as twenty horsepower.
Sorry for no help on the actual tuning process but that’s a little on what to expect as far as how much faster it will be.