Hi Richard,
To save weight, the fastest and cheapest is to use an e-props propeller instead of a duc swirl or flash (if you do not have already a propeller)
Helices E-PROPS : DURANDAL 100 pour Rotax 912S 914
Mine is à 100 M, as i remember the weight is under 3 kg and the saving is around 4 kg
Without painting, i know a savannah s under 290 kg with parachute
This one : Construction de mon Savannah S
You can follow the construction with many pictures
Regarding your proposal "Carbon fibre engine cowl, wing tips, tail and fin tips." it is a good way, if somebody would like to creat this parts, i already know few guys interested to buy thisese carbon components