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Originally Posted by JamesNoBrakes
That wasn't really my point, my point was that if you are "slowing the car" by downshifting WITHOUT rev-matching (manual or automatic), you are basically slowing the car with the clutch material, once it's matched and fully-engaged the clutch the engine compression is what's slowing you down, mainly the pressure would be on the rings I'd think, but not like they can't take it, since like you say mild acceleration would be just as much stress.
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Keep in mind that you re-engage the clutch on a rev-matched downshift just like you do on a normal (unhurried) upshift. It's not like when you're starting up from a full stop at all.
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Otherwise, downshifting WITH rev-matching bypasses the whole "clutch friction to slow down" issue and relying mainly on the engine compression.
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Emergency "help-I've-lost-my-brakes" situations aside, using clutch friction to specifically slow the car down is not why you should be downshifting. Remember that the process of downshifting is not so you can get a braking effect of any amount, let alone a maximum possible braking effect. It's only to match the engine revs to what the next gear wants them to be at, for the speed you're traveling at. Done right, you should hardly feel the clutch re-engage - if you feel it at all.
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I always noticed that my low-compression turbo WRX had crap for engine compression (as in very low, because the turbo makes up for it), so downshifting wouldn't slow you down much, despite the short gearing.
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The Subie in my sig probably has less engine compression braking available than your WRX. It's never felt like its compression effect was inadequate here in flatland S. Jersey, and 4th gear seems to have been low enough to maintain speed down most of the longer and steeper downgrades I've been on with it so far (though I would not be opposed in the least to using 3rd if that's what it was going to take).
Honestly, this whole business of shifting and rev-matching became second nature a long time ago, with little or no conscious thought ever involved until I try to describe it to somebody else.
Norm