Yes, you should replace the cap. If it should fall off the adjusting tee will possibly fall out. The part is 126 987 07 45 and lists for $.70.
The other adjustment is where the cable attachs to the throttle linkage. Ther are a number of different arrangements but basically lengthening the cable housing in effect shortens the cable and visa versa. Shortening the cable makes shifts later. I suspect that making your shifts earlier could improve things but that depends on what happens now that the shifts are firmer. Since kick-down shift points aren't affected by this adjustment shifting earlier doesn't necessarily reduce performance.
Make your adjustments small and try things in both directions slowly (drive for a while in between to get a feel).
Bosch Master, ASE Master, L1
33 years MB technician