Turns out there's third-party pathfinder material that has some pretty awesome formian pictures. I might even replace the warrior's and worker's with them.
Taskmasters have 6 RHD, and chassis-wise, they haven't improved much over warriors. A large boost to wisdom and charisma is the most notable change: apart from that there's slight improvements in strength, natural armor and SR, as well as the gain of telepathy and loss of a natural bite attack.
I almost forgot: taskmasters get two more fun little powers. The first: Dominate Monster at-will as a (Su) at CL 10, with the only limitations being that targets must be no larger than Large, targets are immune for 24 hours if they make a save, and the formian can't dominate more than four creatures at once. This happens to be very good.
Now, people might be reminded of the aboleth, which has a similar ability. However, the formian's is superior in several ways.
The aboleth can only use its ability thrice a day: a formian can just use it every round of combat and kill off the unwanted slaves after the fight. It's essentially a walking save-or-lose, except whoever loses also joins the formian's side.
In addition, an aboleth has to stay within a mile of slaves while no such range limit exists for the taskmaster. Finally, there's the matter of slaves breaking free: an aboleth's get a chance to break free every day, whereas a formian's can only escape by succeeding on ten successive saving throws (and even then it'll already have been under the formian's command for over a week).
But wait: I said there were two abilities. The second is simply called Dominated Creature, and it's a horrible mess of a trait. It's supposed to justify NPC taskmasters having a monster with them, but on a PC I'm not sure what it actually does. Can you choose any CR 4 creature to have permanently dominated? Can you replace it? If the old creature dies, does a new one appear magically? Is the domination Extraordinary? Does it take up one of your four domination slots? For the sake of sanity, I'm going to shelve it with the pugilist fighter and bloodlines, and put an asterisk here.
So what LA to give? I'm honestly stumped. In the end I'm going to go with +3*, but more out of a gut feeling than carefully deliberation.