First of all let me say I love the idea of evolving magic items and I hope you find results you and your party are happy with.

That said, I feel the ranger is getting a bad deal. They will likely eat a lot of damage coming their companion's way, I'm guessing, to keep it alive and active. And while sense link is fun, blinding/deafening yourself doesn't seem necessary -- we are talking about a +3 level item here, that doesn't seem like enough of a bonus. And I can't think of a lot of spells other than cure wounds a ranger would cast with range touch. There might be some, I just can't think of any off the top of my pelt. Head! I meant head.

Everyone else is getting significant combat bonuses. The Ranger, at least to me, doesn't feel the same. Have you considered the bonus also including +1/+2/+3 to the animal's attack and damage rolls? Or, failing that, AC and saving throws? The monk, barbarian, and rogue all seem to get that.