A borg founder won't have the capability to shapeshift its cybernetic implants into existence
But it would be able to *hide* them under a 'Founder skin'.
And, even if the assimilated founder would have all the abilities of a free one, he would be far less dangerous than you seem to think - he won't even come close to "assimilating an entire world by himself".
How do you know?
Assuming a Borg Founder can do what a non-Borg Founder can do - imitate the appearance *and personality* of anyone it wishes - it could sneak up on people and assimilate them before they knew what hit them. No one would ever know that it's a drone, so they couldn't target it and wipe it out. (At least with normal Borg, they can't help but "announce" their presence, since it'd be obvious from just looking at them that they're Borg.) That sounds pretty dangerous to me.