Well, since you are a recreational sailor, I would go with a 520 100% carbon mast.
It will be expensive, no doubt, but not nearly as expensive and fragile as a 550 cm race mast. The 520 will also be considerably lighter in weight.
I'm pretty sure you won't get much gain with an 11.0 m2 rig, over the 10.5 m2 rig, and I'm also pretty sure you would notice the difference between the 9.8 and 10.5 m2 sails.
So, go for the 10.5 GTX on a Gaastra 520 cm mast.
Hope this helps,