The weapons table says they (and chainfists) double the SB that gets added to damage, and their text description says that power fists increase the wielder's unnatural strength multiplier by one.
Now, applying both of those would mean that on a marine with 50ish strength a hit would do 2d10+34 damage per hit, before taking into account any other bonuses from talents and weapon quality.
[2d10 + ((5*3)+2)*2]
Normal SB 5 from 50~ strength, tripled from Unnatural Strength x(2+1), plus 2 from armour (+20), all that doubled due to powerfist description.
And well, this seems rather excessive. While I can totally see it as something that'd wreck a tank, it renders other melee weapons irrelevant as damage dealers.
Of course, you could also assume that the "doubles strength" is an extension of the "increases unnatural strength by 1" description in the text, but going by the newest Living Errata that'd mean that Thunder Hammers are better than Power Fists in every way, since they cost the same, yet do more damage and add the perma-stun concussion ability in addition to being more versatile (as they allow the use of the hand outside of combat).
So, uh... thoughts?