The "Bob" is often mentioned in gunsmithing books as being a good caliber to rechamber the Swede's to, along with 7X57, and .35 Remington. Quite a few do not recommend the .308 & family for this action, BUT alot have been safely converted and used. Also, it is worth noting that Kimber at one time aquired a bunch of surplus Swedes. Those with good bores were "sporterized" with a molded plastic stock and scope bases. Those with poor bores were rebarreled with .243, .308, and maybe .22/250. So it appears that if the action is sound, the conversion, properly done, is o.k.
I don't know what pressure the .257 plusP are loaded to..I'd compare them to the .308 factory loads before using them.
Mark