today it looked very windy so i only brought smaller 52wide 80litre with me.the wind dropped 10/20 knots averaging around 13knots. i had an absolute dog of a day with 5.0m. however had i took my 58 wide 95litre larger board i would have had reasonable day.point being a 5.0m rigged full on a 58 wide you can sail mostly powered up in around 12/16 knots for 70kg sailors. a smaller 80 litre board used does as i found today make it a grovel.the other advantage that i did not mention in original post is in stronger wind 20/25 a 5.0m will have higher topend speed over 5.8m or bigger as control is greater esp on reach. in 20knots plus with 5.8m i immediately sail upwind in gusts.this is why i tend to prefer 5m over 5.8m i never can never be too sure when wind will pick up. in slalom comp in 12/16 i do agree larger sail will give you clear edge over a 5.0m.