Wouldn't about half the 300 to 400 fps difference be due to higher pressure loads and barrel length?
On their website, Lapua has a 6.5x55 139 grain load for modern firearms that produces 2842 fps for a 139 gr bullet out of a 26.4" barrel.
The 6.5-06 is great at what it is - a specialty long-range round, but apples to apples, based on data I've seen, it gains about 150 or so fps on the 6.5x55 loaded to equal pressures with the same barrel length. I think that 150 or so fps falls in with one of Mule Deer's ballistic laws of 1/4.
