Ahhhh....an original GO! What a classic board!
Mmmm... I'm not sure what an "X3" Powerex mast is.
Normally that would be a Z-Speed (100% carbon); Z-Free (75% carbon); Z-Wave (55% Carbon) or the NRZ (30% carbon).
Powerex masts normally have fairly uniform bend characteristics
throughout their mast line so the significant variables would be the
weight and price.
As to your question, my experience has been that the Retro's can handle about a 5 knot range pretty easily. It will be a little underpowered at 14 knots, and you'll definitely find you have too much sail at 19-20 knots.
But why be "limited"? Get an adjustable outhaul (a simple one was provided with your sail and should be in the pouch in the sail bag. This would allow you to "tune" on the fly and get maybe a knot or 2 on the low wind end of the range (if you slack the adj. outhaul off alot) and maybe a knot or 2 on the top end as well (by simply flattening out your sail by tightening the outhaul.
And, you can add quite a bit more downhaul and increase the range even more.
At 70 Kg,/154 lbs. you should be able to pump the GO onto a plane in 12-14 knots somewhere, depending on your skills and the "consistency" of the wind. I've sailed the 6.5 Retro in gusts over 20 knots, but I have plenty of sails so at around 18 knots steady I get out the 5.5 m2 Retro.
Yes, I think most modern sails can easily handle a 5 knot (maybe even as much as 10 knots in the gusts) range of windspeeds and be comfortably powered throughout that range.
Bigger formula sails, with a flatter entry and more twist dynamics can handle as much as 12-15 knots wind range, but it's not fun to be that far overpowered.
You need to select sails based on the type of sailing you do most often, and the Retro is right there in mid range of this. You have flatter, simpler free ride sails and Freestyle sails that don't have as much range and cannot be tuned through a wide range, and on the other end of the spectrum you have the full race sails that have a tremendous range, and a huge tuning range, but they are more complex and a little heavier, and definitely more difficult to rig.
You are a light weight, so you can get your GO board going pretty early, and with good technique, you can hang on the that same
Retro (make it easy on yourself and do some tuning with the downhaul and an adj. outhaul) up to around 20 knots.
Beyond about 18 knots, your board is going to start getting "too big" and changing down to a smaller fin and rig will allow you to go to maybe 22-24 knots. Beyond that is unrealistic for the GO board.
You can certainly sail the GO in more wind, but it won't be fun and getting a smaller narrower board and smaller rig/fin will bring the fun factor back.
I hope this answers yo9ur question.