Hm. A Reflex save to attack the right tentacle or something? They're all whipping about wildly, from the description. If you wanted to ignore that because it's too complicated, I'd be fine dropping it, but it's something to consider.
I don't know, in order to do enough damage to sunder, it'd be hard to pick. I can see the logic for a Ref save or Dex check, but I'm just as happy with a 50% chance of either.
An opponent can attack a burrower's tentacles with a sunder attempt as if they were weapons. A burrower's tentacles have 10 hit points each. If a burrower is currently grappling a target with the tentacle that is being attacked, it usually uses another limb to make its attack of opportunity against the opponent making the sunder attempt. Severing one of a burrower's tentacles deals 5 points of damage to the creature. A burrower usually withdraws from combat if it loses six or more(?) tentacles. The creature regrows severed limbs in 1 day. A severed tentacle results in the loss of one of the burrower's tentacle or plasma needle attacks (50% chance of either).