With the rod, I summoned a CL40 bear on my Sorcerer and used it in parallel with his CL 52 Mirrorgolem (Mastery III)
Level 40 Bear has (unbuffed) 4,760 HP, 104 AC, 104 AB, 30% Immunity all, no passive Regeneration whatsoever.
Has Improved Critical, Weapon Specialization with the creature weapons, but these feats seem to have no effect. Animalistic Power increases AC to 110 by pumping Dex, and this can be increased to 113 or so with Druid dodge buffing spells.
A CL 52 Mirror Golem in contrast has 3,350 HP, 128 AC, 116 AB, 30% Immunity all, 80 Regeneration and 60% conceal.
Tested on EX3 Lions (115 AB) in smuggler's. Bear gets badly beaten up, but Mirrorgolem barely gets scratched.
Testing on Thumper (14 AC, no immunity) had the bear pegged at 2.2 K DPR and the Golem at 1.25 K DPR. As both companions hit for similar numbers on a per-blow basis, the Bear is clearly getting some extra attacks from multiattack that the Mirror Golem is not. This might be a problem when balancing the different companions, as different creatures have different multiattack patterns, and nobody seems to know exactly what companion gets how many attacks. Would recommend dropping the multiattack feat for this reason.
AB from both pets is far too high. A target unbuffed AB of 100 for the Mirror Golem and 90 or so for the Bear would put them more in line with player characters.
Toughness wise, the Mirrorgolem is ridiculously tough due to the combination of solid HP, decent regen, insane AC, targeted magic immunity, and 60% conceal. The bear takes a huge beating as it lacks any conceal and suffers a much lower AC. Without passive regen of any sort, the bear absolutely needs a regeneration spell to keep alive. This spell gives it 476 hp/6s at base.
Suggestions: Lower the bear's HP to 3.4k, but give the bear passive regeneration set at 4% of it's HP (136/round at 40). Casting Regeneration on this bear yields (136 + 340 = 476) hp/round, identical to the current incarnation with regeneration. The passive component makes a bear companion not utterly useless for a ranger. An additional Vamp regen of 1 per 4 levels (max 10) on its attacks could also be considered to make up for the HP loss.
AC wise, a slight buff of +5 or so on the Bear (109 base, 118 full buffed - maybe via Armor Skin III?) and a nerf for the Mirrorgolem (118 at CL50?) would put them more in line.
Nerf Mirrorgolem conceal to 50%, lower AC as stated above, and lower HP to 2.5k or so while keeping Regen at 80. This makes it weaker than the bear in toughness, as it should be.
Bear has fairly awful saves at 33/35/14. A +12 saves buff pushes this to 45/47/26, which is still too low vs most content. Would suggest increasing saves by somewhere between +10 to +20 all to keep it in line with 40 players.
Bear also lacks death, mind, KD, Uncanny, Evasion, and level/stat drain immunity, which limits its applications in combat.
On death, the golem/bear spawn a loot bag with 3x Universal Immunity shards. They disappear on player pickup (curses!), but you might want to make it so it doesn't drop the items at all.