RE: Board and Sail
Never having sailed in Perth (I almost got to go there once, but the ship moved to Thailand and I met Svein instead, so the outcome was quite good overall) I really don't know quite what to tell you.
Suggest you make a trip to the windsurf launch area there in Perth and have a look around to see what others are sailing.
If you are hooked in, in the footstraps, and planing, with 15-25 knot conditions I think the suggested GO 139 is a bit too big and wide.
Flat water in 15 knots is one thing, flatwater in 25 knots would have to be something quite different.
If you aren't going to be sailing in < 15 knots then you won't need the width and early planing you might get with a GO 139-170.
I'd think a mid size Carve would be bettr up to around 20-22 knots with a 6.0-6.5 m2 rig which shold get you going nicely in 15 knots at your weight.
The NCX is an OK sail, but you might want to get something with a little more "jiuce" (draft in this case) like the Gator or perhaps the Overdrive.
If you look for a quiver of say 7.5/6.0/5.0 you should have your stated wind range pretty well covered on a mid size Carve (122/111).
If you get more 25 knot conditions than 15 knot conditions, you might even want to look at something smaller.
Like I said, check out the local scene and see what other sailors are using, maybe even rent a couple of different size Carves and the smaller GO boards to see what really works best in the Perth conditions.
Hope this helps,