Well I have got out on the 6.4m a couple of times now so here are my first impressions.
Stats
Mast required – 460(imcs range 25)
Luff – 461
Boom - 199
Rigging
Rigging is easy as the cams stay on the mast no worries it rigs just like a RAF sail

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On the water
The 6.4m to me feels like it’s the perfect match for a standard WA sea breeze. Its is just so stable and as Tushingham say its top end is limitless.
I sailed it in fully powered up conditions at hardies Run in 15-25knts and found it cranks along without any fuss. I was constantly getting 30+knts without getting any decent good gusts.
I use an adjustable outhaul and this really gives the sail a huge range. At times the wind dropped out to less than 15knts and I could easily bag the sail out and pump onto the plane no worries at all.
All in all I think this size in the X-15 will be my most used sail in their range. I cannot recommend them enough.

Cheers Bender