You seriously need to understand the max weight of the none lifting parts, you can’t use the weight available as water ballast in the wings in your calculations, as being in the wings it does not
increase the bending loads of the spar, and you will be overloading the lift pins on the fuselage. The suggestion to put extra weight in the wings by fitting extra ballast capacity is also a very dangerous concept as you will he increasing the negative g loads which are only half the positive g rating. The max weight is there for a reason.
Otherwise, it’s a good plan. Bringing the profile to accurate form is your best way to improve performance by far, as Slingsby did a very poor job of profile accuracy.