Yes, the lower gears make you accelerate faster. But if you floor it in D, unless it's the dumbest auto in the world, it'll start you in first, then proceed to second, then to third, etc. just as if you were "shifting" it yourself. What, do you think that leaving it in D will make it not use the lower gears? It'll just start in 2nd or 3rd no matter what you do? Of course if you punch it in D it'll use the lower gears, that's why it's an automatic. It only has 4 speeds in the first place, it's not just gonna decide not to use the "lower" gears. At any rate, it has a torque converter and therefore, to me, isn't worth it. Much less power to the ground, much less involvement, and still has SLOW shifts and longer gears. It's like having sex with a quarter-inch thick condom; you could do it, but it would be less fun, less involving, and would take a lot longer....
Trust me, if it helps, it's not by much. Car and Driver tested a bunch of manu-matics a couple years ago, cars like the Boxster and A4 that have "Tiptronic". They did 0-60, 1/4 mile, everything, shifting it themselves and then leaving it in D. None of the cars were much faster with shifting it, just as many were slower, and most were exactly the same. And those are cars with "Tiptronic" systems, not a generic auto gate.