I talked with Stefan today, he's at his own X-House in northern Sweden, still enjoying his holiday. He feels strongly we should add as much insulation as we can while we build; his house has the full 300mm. Rutger called me back and he's passionate about adding insulation, too.
Janne and I talked this evening and asked him how much I'd save if I cut the order back to thinner insulation. Much of the cost is transportation, so I'd save a few thousand kronor, which is nothing to sneeze at, but I've decided to keep my original order of 300mm insulation. I may regret this short-term, in terms of cost and trouble, but long-term, nobody regrets having a really well-insulated house!