The 6.5x55 is showing lower velocity than some of the smaller cased short action 6.5's because SAAMI holds it to a much lower pressure limit. You will find that to be true of all of the old mauser cartridges. The old mausers used the lower pressures and 29" barrels to acheive the performance that made the 6.5x55's legend.
If loaded to the same pressures in a modern action, the Swede will outperform the smaller cased 6.5's because it has more powder capacity.
Here is an example of loading data for the 6.5x55 in a modern action, 26" barrel:
Compare the 139g Scenar data to the 6.5 Creedmore Hornady factory 140 AMAX load @ 2710 and you will see what I mean.
When you compare modern data for the 6.5x55 to the lower spec data common in the U.S. (because it is written for the weaker small ring mausers), you will see larger charges of slower powder and longer cartridge OAL's in the modern data.
The AI version of the Swede reportedly narrows the gap between the 6.5x55 and the 6.5-284 considerably.
Where the old Swede mausers are concerned, they are sweet little rifles. Their performance is limited, though, both because of their metallurgy and because they use weaker small ring actions. In reality, the milsurp Swedes won't save you any money because they will require a lot of work to make them into useful sporter rifles. Still, if you have a soft spot for the old mausers, as I do, you will no doubt enjoy owning and shooting them.
If, however, you are looking for a max effort 6.5x55, you need to at least look to the '98 pattern mauser as a basis for your build. A newer manufacture commercial mauser action is a better option still because you get better steel and the action is already in sporter configuration. Of course, using more modern action designs like a Savage or a Remington would probably be considered by most folks to be the best way to go.
FWIW, I have two 6.5x55's, one on a 98/22 mauser action and one on a newer Zastava commercial mauser action. I also have a 6.5-284 Norma built on a Ruger 77 Mk II action and a new production Winchester Model 70 in .264 Winchester Magnum.
They are great rifles. I love my 6.5's!