I'm with Bart B. this guy sounds like a gun store know it all. Strong opinions. but some wrong conclusions.
As far as lethality goes, it comes from Accuracy (the 6.5x55 general has it), Penetration (the 6.5x55 has a high sectional density for good penetration), and finally disruption (how big of a wound channel can the bullet expand).
If you look at a 140gr 6.5mm Sierra Game King launched at a sedate 2600 fps from a 6.5 Swede, compared to a 7.62 150gr Sierra Game King launched at 2800 fps from a 308 Win, the ballistic drops are within 1.3 inches at 300 meters, and energy remaining at 500 is actually higher for the 6.5 Swede.