By BusinessWeek's admittedly rough estimate, offshoring may have created about $66 billion in phantom GDP gains since 2003 (page 31). That would lower real GDP today by about half of 1%, which is substantial but not huge. But put another way, $66 billion would wipe out as much as 40% of the gains in manufacturing output over the same period....There are a number of linked stories and charts. Here is a follow-up.
In terms of trade policy, the new perspective suggests the U.S. may have a worse competitiveness problem than most people realized. It was easy to downplay the huge trade deficit as long as it seemed as though domestic growth was strong. But if the import boom is actually creating only a facade of growth, that's a different story.
The one thing they don't mention by name is shadowstats which had a good primer on the GDP in 2004. Note that John Williams of Shadowstats is not convinced by the BusinessWeek analysis which is separate from his issues with the numbers. I find a lot of the BW argument compelling with their evidence that the sectors most affected by out-sourcing and imports have the most unbelievable inflation and productivity figures.