Are there different experiences of equanimity? Specifically, is there a difference between equanimity as a corruption of insight and the insight stage "equanimity in regards to formations"?
On an ultimate level, equanimity is simply a feeling associated with a mind state; it is the quality of the mind state itself, independent of the equanimity, that determines the ethical nature of the state. 

For example, lobha (greed) can arise with either a pleasant or a neutral feeling, both of which are unwholesome of course. Purely delusional mind states (moha-mūla-citta) arise only with equanimity - there are two of these, worry and doubt, and they are both unwholesome.

On the other hand, beautiful (sobhana) mind states also can arise with either pleasant or neutral feelings, and they are purely wholesome.

In the middle, there are the six sense resultant consciousnesses that are always accompanied by a neutral feeling (except physical, which can be associated with painful or pleasant feeling, IIRC). None of these are wholesome or unwholesome.

As to your specific question, equanimity as a defilement of insight is not necessarily qualitatively different from equanimity at the 11th stage, since the defilements of insight are none of them "bad". It is only the clinging to the states as the path to enlightenment that causes the true path to be neglected, hence "sullying" one's insight. Ostensibly, the equanimity itself is a positive sign of progress, but it is not the path - the four satipatthana are.

That being said, the equanimity that arises prior to the eleventh stage of knowledge, even if based on insight (it is often based rather on tranquility), is immature and unstable, whereas at the 11th stage it is lasting and firm, unshakable, and universal. It is the culmination, or maturation, of any insight-based equanimity that may have arisen previously.

As to equanimity in general, the Visuddhimagga (IV.84) lists ten types of equanimity, some of which are of the same quality:


six-factored equanimity - equanimity of an enlightened arahant
equanimity as a divine abiding - as a specific samatha meditation practice
equanimity as an enlightenment factor - having to do with relinquishment of desire
equanimity of energy - neither lax nor overexerted energy
equanimity about formations - 11th stage of knowledge, having overcome hindrances, regarding all formations as impermanent, suffering, and non-self
equanimity as a feeling - refers to the basic quality of neutrality common to many mind states
equanimity about insight - investigating states with objective intent
equanimity as specific neutrality - the balance of mental qualities as neither deficient or excessive
equanimity of jhāna - through being absorbed in an object to the point of abandoning pleasure and displeasure
equanimity of purification - describes the purification of the mind of partiality through various means.


Most of these just refer to different ways in which the concept of equanimity is used in the suttas and abhidhamma; in ultimate reality, as stated, most are called equanimity simply due to their shared quality of being associated with a neutral feeling. The difference is rather in their other specific qualities like wholesome or unwholesome roots, and the cause that brought about the equanimity - insight, tranquility, delusion, etc. More specifically, numbers 1-3 and 8-10 are all qualitatively the same as each other, as are 4 and 6 with each other. 5 and 7 are unique.