If u rev-match downshift, the blip wasn't enough for the shift. Are you destroying the synchro, the clutch or the gears in the transmission?

 

It's hard on the clutch and gears. Syncros are only used to sync the layshaft with the driveline.

 

I think you're asking if erroring on the lowside of the revs while rev-matching will cause any problems? The answer is not really. When you let out the clutch, the engine basically gets forced up to speed. Yes, this puts some stress on your driveline and your clutch, and no, it doesn't affect the syncros at all.

Its generally better to error on the high-side of the revs when rev-matching as that extra engine speed will bleed off quickly.

 

Thanks for the feedback, I was kinda worried I might destroyed the gears while getting used to heel and toeing in the car.