from starboard 8'10 widepoint 143 liters ...going down to Mistral 8'6 130 liters.
body weight 85 kilos .advanced surf skills.
is this enough/big difference?

Is there a difference in width between these boards?
That will make a big difference as we'll .

It isn't whether you go up or down in litres that is important but how that volume is distributed is THE most important factor. I can go from the Starboard 8'5x29 pro (108l) to the Air born 7'10x31.5 (98l) and guess which one is more stable?

I've gone from 174 litres to 136 without any difficulty at all at 9okg ......as above...all to do with the board's design and distribution.

Mate I'd be looking for something in the 100 to120 litre range, reduce width and volume ,focus on shape and life will be sweet