I drive a Mustang GT, and I love the deceleration "pop" that you get when the engine revs climb without the accelerator being pressed, so as one might surmise, I tend to downshift to a stop at lights and stop signs to hear that "pop pop rumble". Generally it's from 5th at 40 mph to 2nd, which is well under redline, but it's a large enough increase in engine RPM to give a few good pops. I haven't noticed any signs of additional wear and tear, but I figuredd I should ask those more versed in this sort of knowledge than I about it. Is there any significant increase in wear and tear on any of the drivetrain components from doing this? Things I can think of are maybe the clutch, the transmission output shaft seal, and possibly the gears themselves either in the transmission or in the rear end, but I think those last two are unlikely. I'm thinking over time I might end up clogging a cat as well, but I'm not exactly sure how quickly that would happen either.
The synchros will be forced to bear the brunt of downshifting like this
The synchromesh gears are gears inside the manual transmission designed to help the engine transition smoothly to its new RPM.
They will wear out eventually, but the wear and tear will be accelerated by downshifting without adding throttle to help the engine match its new speed.
The act of adding throttle helps by increasing engine speed, allowing it to accommodate the ratio of the lower gear without loading the synchros as much. If the throttle input is such that the engine-to-driveshaft speed ratio corresponds to the gear ratio (rev-matching), the synchro has nothing to do (zero load).
A worn synchro will result in a crunchier, more notchy gearshift. It makes shifts more difficult to perform. In such a case, refreshing transmission fluid will only have a temporary effect. The only true remedy is to replace the worn synchro, which involves pulling out the transmission from the vehicle and disassembling the gearbox.
Downshifting without throttle input will accelerate wear on the clutch as well, but not to the same extent as what the synchros will experience.
I don't think downshifts will result in clogged catalytic converters.