The speed is detected from the rear ABS sensor, so gearing changes make no difference at all.
Changing sprockets is always an interesting experience. If you change the rear sprocket then in first gear you get (new/old)*100 percent more power (as power = torque*rpm) but you run out of gear the same percentage sooner quicker.
As a ridiculous example, if you fitted a 57 tooth rear (or a 13 tooth front sprocket) you would have a new gear. 2nd would be the same as 1st was, 3rd the same as second was, etc. From this you can see that you can't possibly gain any power from changing sprockets, but your new first gear would have loads of torque but very little speed, so you'd have to change up a gear 30% sooner.
That means if your bike can pop a wheelie when leaving the line there's no point gearing it down in 1st. But, if you find you're having to slip the clutch too much then you should gear it down. When you gear it down you will get poorer fuel economy, as you increase the revs in every gear, but it only really effects freeway cruising as your revs will be higher for any given speed.
It's a relatively cheap mod, and easily reversed, so go for it
Just be aware of what's happening, and make sure it's the effect you want.