RE: light wind sail for original go
What size sail do you currently use?
The original GO can easily take sails up to around 10.0 m2.
A good free race 9.5 m2 (SW Retro, NP V8, Severne Gator) would give you about all the light wind performance you can get from the original GO.
A 9.5 ahould be good on the GO to around 12-14 knots where you might want to change down to a 7.5 -8.0 m2 rig.
If you only want a smaller sail, look at somethign really powerful like a Severne Glide.
Youir Powerex ZWave mast is not going to work too well on larger free race sails so you will need to get another mast (hopefully the "best mast" recommended for whatever sail you purchase).
Hope this helps,