With a shift kit, you will still get banging when downshifting (possibly worse). You working the tranny by manually shifting like you are and using your trans as your brakes instead of the pedal on the floor your trans temps will sky rocket. I have a TransGo shift kit in my trans and I downshift pulling a mountain because I am tired if it going back and forth into gears. I'd rather keep it in a lower gear so just cruise up. But with this said, if I downshift with my foot off the gas at a higher speed, it bangs.
I don't think a shift kit will help you. Drive your car normal and see if it stops... I don't hear any complaining of hard up shifts just manual downshifts, unless I misread something..
