at your weight 54kg, moving straps to very back position would produce too much board lift, especially in stronger wind. i am another 12kg heavier, i have my backstraps as far forward as possible.
this helps to stop the board from taking off in gusts.
ask different sailors and some will say moving straps back is faster, a few will say moving straps forward is faster.
at my 66kg i try to reduce the lift of board as much as possible.
straps forward,weight in harness forward to nose, smaller fin,lower boom and high hook height all help.
a 5.8m at 54kg is quite large. a 4m + 5m will be faster for you if the wind is decent.