I’ve done a fair amount to my SL5:
Fizik Carbon 3D print saddle
Carbon bars
Roubaix tape
BBinfiniti ceramic bottom bracket
BBinfiniti larger pulley wheels
Garmin 1030+
Bontrager lights
Varia
DuraAce rotors (cause black looked cooler)
Bontrager Aeolous Pro 37 wheelset
Bontrager R3 tires in 28
Stages left crank power meter
IMO, the must haves are carbon wheels, Blendr light and computer mounts, PM, and carbon bars. Bottom bracket if problems.