turning into a shark or scorpion are just things that they should be allowed to do flavor-wise. They are perfectly free to make up new forms.

And I'm not balancing it against t1 casters. My points is that a tier 2 caster can easily have all of their utility with level 2 spells, and a few level 3 spells. They don't access any of them earlier, they don't get access to higher level abilities, and they actually end up with less action economy compared to summoner until very high levels. Summoner can cast evolution surge on their eidolon, and in that same round the eidolon can full attack. Evolution surge increases its capability, and is sufficient to add pretty much any situational ability you need. And the summoner can do all sorts of other things that a shapeshifter can't.

This comes in well behind any tier 1 or 2 caster. I'd estimate it at tier 3, which is a perfectly reasonable tier.