I'm a little confused by this statement. 3555 and 5575 are two diffrent shaft sizes. The 3555 look like they'll be with the range for spine but If you crank the bow up any you'll be on the frindge of being underspined. The 5575 shafts will be a bit on the stiff side for your set up.

I'd trust the GT site. A foc in the 11 range is fine. Mine is a tad bit lower at 10.18% but arrow flight is good.

I've shot cx and gt arrows and have settled on the gt pro. Bolth are great arrows but I think the durability of the GT shafts is damn near impossible to beat. They are truly a rugged arrow.