It is not the bullet weight that determines the twist ratio. It is the bullet length, caliber and velocity that determines this.

http://stevespages.com/page8e.htm

I'd also look at neck tension, but poor shooting at full pressure makes me also suspect uneven locking lugs or partial contact on a recoil lug (doesn't Tikka have that funny round locking lug arrangement?). Anyway, something is not bearing evenly, seems like. JMO, Dutch.

It is not the barrel twist. I have a 30 cal. Kostyshyn barrel that shoots 150, 165, and 180 bullets all MOA with different brand bullets, powders, etc.

Just for test: try a moderately loaded 180 bullet over a medium burn rate powder, like IMR4064. See what happens.