One glaring point I felt should be addressed is your question about whether an automatic transmission is 'designed for downshifting to slow down'.
The answer is NO, N-O transmission is designed to be USED to SLOW the car down!
Some transmissions(those in vehicles that haul heavy loads) have trans brakes, but that's not what we're talking about here.
YOUR BRAKES are the ONLY parts/system on your car, that is 'designed to be used to slow the car down'. If you use your manual trans to slow your car down, you're simply wearing out your expensive clutch-and possibly internal VERY EXPENSIVE transmission components, rather than the INEXPENSIVE and EASY TO REPLACE brake pads.
If you use your Steptronic-type transmission to slow your car down, you're risking not only your internal gearing, but also your differential and ring and pinion.
I guess people got the idea that using a manual trans to slow the car was a good idea from watching racing drivers back-shift as they're slowing. You'll also note that they also REV THE ENGINE to match the rpms on the back-shift. This action has absolutely NOTHING to do with slowing the car down. As a matter of fact, if the driver makes a mistake, and doesn't properly match the engine speed on the back-shift, he risks losing control of the car. When slowing down at high speed(s), most of the weight is on the front, thus VERY LITTLE weight is on the rear. If the driver does ANYTHING that aids in the rear tires losing traction(i.e. mis-matching the rpm's upon back-shifting, resulting in the rear tires spinning faster than the rpms can match-OR- USING THE TRANS TO SLOW THE CAR), the back end of the car will remove itself from the control of the driver, resulting in a plethora of possible problems, NONE OF WHICH the driver can do a whole lot about, since he's got to get the car slowed for the corner. His only real option is to let off on the brakes so some weight transfers off the front and onto the rear(and to also get the rears to speed up a little). Problem there is, again, he's got a corner to slow down for, so whichever way you slice it, he's screwed.
Unfortunately I took the very long way of simply stating the fact that your BRAKES are what you use to slow the car down. Your TRANSMISSION is used to simply put the power down in the MOST controllable manner. Racing drivers DON'T use their transmissions to slow their cars down(well...at least when their brakes are working). People mistake them doing that, when in fact they're simply matching the engine speed with the wheel speed, in order to keep the chassis under control as he slips the trans into the proper gear to give him the best drive possible out of the next corner.
I think I just typed my way through Tosh 2.0.
I hate me!