From my experience, the best accuracy and tightest groups on the FX Impact comes with leaving the barrel/shroud free floating. It was designed to be free floating.
A year ago, i made the mistake of wrapping a foam sleeve over my Impact's carbon bottle as protection. The thickness of the foam sleeve caused it to barely touch and exert just that small amount of pressure on the barrel shroud. This immediately resulted in a POI shift of 2.5 inches higher at 30 yards and caused the groups to open up significantly. I couldn't believe it because the foam protector sleeve was "barely" touching the shroud, if even. But if you consider barrel whip during a shot cycle, the foam sleeve definitely messed with the harmonics and exerted unpredictable external forces on the barrel/shroud.
I removed the foam sleeve and the POI and more importantly, tight groups came right back. From personal experience, I doubt some of the above solutions trying to lock down the Impact's barrel/shroud would work in the long run because while it might reduce some POI shift initially, it could cause group sizes to open up and subject the POI changes to other external forces(like temperature change, etc...) since it is no longer free floating.
One just has to be extra careful with a free floated barrel rifle and avoid leaning or knocking or allowing anything external to affect the harmonics of the barrel.
This also happens even in long range precision rifles in the powder burner world(start from 4min mark):
https://www.youtube.com/watch?v=PEDs3dP2Ck0
