I actually quite like the voidmind template: illithid-created infiltrator beasts with sentient tentacles? Hell yeah.
Let's review the bonuses it gives: +4 natural armor, DR 5/magic, and 10+HD SR are a trio of reasonable defensive abilities. As far as stat boosts go, there's +4 strength and constitution, +2 dexterity and intelligence, and -2 charisma. They're also immune to acid, mind-affecting effects, ability damage, ability drain, and energy drain.
They also grow a retractable semisentient tentacle of slime. Its damage is hardly impressive (1d6 for a medium host), but it has Improved Grab and Constrict and most importantly attacks independently. To expand on the last bit: the tentacle can attack at full BAB regardless of what action its host takes (though you can't double down on its attack) and it can grapple a creature without penalizing the host. It's also got reach, which is neat as well (sadly, the ability doesn't elaborate on how opportunity attacks work with the tentacle).
Once per day, voidmind creatures can spray psionically charged slime at opponents. It deals some mild acid damage (1d6/level) and debuffs Will for a while, in addition to stunning foes for various rounds. It's pretty huge, in terms of action denial, but the usage limit hurts.
Furthermore, voidmind creatures get Alertness, Combat Reflexes and Great Fortitude if the base creature meets the requirements. Given the lack of requirements for any of those, I'm not sure why the last bit was even included. They also receive a racial bonus to bluff, escape artist, and intimidate.
The biggest issue with this template is Mind Flayer Host, though. This ability allows the creature's mind flayer creators to know certain things about it, and also allows them to see through its eyes and Dominate it remotely. Obviously, having a trio of mind flayers of incertain psionic ability use you as a walking power funnel can make gameplay slightly unbalanced.
The simplest solution to this: assume the voidmind creature's mind flayer creators are dead or gone, which essentially removes this ability. It's not dissimilar to the approach asterisk-ratings tend to take (and I'm considering adding one just for this ability). That way, not only is the voidmind creature no longer a source of high-level psionics, the DM is no longer capable of taking over the character at any point for any reason.
Disregarding this final problematic ability, I think +2* is fine for the voidmind creature. It gets some stat boosts, useful immunities, good bonus feats, a free extra attack per round, and a good 1/day save-or-suck. That's on par with stuff like half-dragon, if a bit more unfocused.