I've done both the COW method and the bullet jam/shoot method. COW seems to take a case to about 90+% formed, the new shoulder is still a bit rounded in my case(s) though. I create a false shoulder using a .25 cal expander and then neck size back down to 6mm just far enough to be able to close the bolt (with decent resistence) I use 8gr - 8.5gr. of Power Pistol or Herco powders, then fill with COW to the bottom of the neck, topping it off with your favorite paper towel/toilet paper wad to seal it all in.

Or, I've also sized my cases with a .265" bushing and seated any misc. 6mm bullets I have so they jam hard into the lands. Then load an appropriate 6BR powder charge and fire away. I get a better/sharper formed shoulder this way but, as you mention, it's putting additional rounds down the barrel. Lots of guys report good accuracy while shooting club matches and simultaneously fireforming.

While both methods have their own appeal, I would def. do the COW if I had tight necked chambers. Fire then neck turn.

-Rick