Many thanks for your opinion about X-Cite and Helium, it is very helpful for me. I know that 8,5 sail can't work as 10,5 in all conditions, but similar phrase I read on boardseekermag.com site. Of course they wrote it about light wind only: "The V8 Helium really does get you planing early and offers light wind performance that would match a sail 2m bigger.". Maybe...
Explain me please why do you claim "It's for sure not the best sail for formula"? I have problem with my backbone (temporarily, I hope) and looking for solution for light wind, normal on my spot. Actually I have only formula board, but can't sailing with big sails. Light sail with high power in light winds would be perfect answer.