Found this thread earlier. Documents a lot of the feats displayed by the various primarchs throughout some of the novels.
http://forums.spacebattles.com/threa...thread.235897/
So how powerful are they exactly? I looked at this to compare them to marvel superheroes.
http://marvel.wikia.com/Strength_Scale
Vulkan can lift a baneblade tank with his hands and those suckers are 320 tons. I can see primarchs as being able to go toe to toe in a fist fight with the incredible hulk.