Hello Jhananda.
Was the Buddha a "super-arahant" compared to other arahants?
This is a good question. From the sttuas we see that the 4th jhana is defined in terms of being free of suffering (adukkha). It is being free of suffering that defined the 4 Noble Truths; and what defined an arahant was not just being free of suffering, but being liberated from the 10 fetters. What we have found both from personal experience and the suttas is, one experience of th 4th jhana does not free one from either suffering (dukkha), nor the fetters; however, developing one's contemplative life to the point that one can meditate consistently to the 4th jhana definitely does free one from both suffering (dukkha), and the fetters.
However, what characterized Siddhartha Gautama, and what made him a Buddha, was his ability to recognize the significance of all 8 stages of the religious experience, as well as the fruit of the contemplative life. So, to me, what makes a Buddha, Avatar, Messiah, Christ, is the ability to see the whole package, not just a part of it; and Siddhartha Gautama saw the whole package. It is possible that other mystics, such as: Patanjali, John the Baptist, Jesus, Mohamed, Rumi, Kabir, Teresa of Avila, and others, might very well have understood the whole package, but had no student capable of understanding it, or recording.
The suttas describe all kinds fantastic supernatural feats. Do arahants possess supernatural abilities?
It depends upon what you mean by "fantastic supernatural feats." If you mean; levitating, walking on water, parting the seas, raising the dead, turning into a rainbow, and other violations of the laws of physics, then, no. No one can violate the laws of physics in the physical dimension.
However, if you are speaking of the non-physical fruit of the contemplative life, such as: the 8 stages of the religious experience; OOBEs; virtue/kundalini; being free of the addictions/fetters/sins; intuition and insight; sensing non-physical phenomena, such as the charisms, etc., then there are lots of fantastic phenomena, which we discuss on a regular basis on this forum, and I help people to sense them.
Why is it that you always refer to the Buddha by his common name, Siddhartha Gautama?
According the the suttas, Siddhartha Gautama, was not the only Buddha to have come along, nor the last; therefore, I refer to him by his name, so that I do not reinforce the rayification of Siddhartha Gautama, nor Jesus, as the only begotten, or last messiah/buddha, etc.
Buddhahood, or messiahship, is available to anyone who lives the lifestyle that produces all 8 stages of the religious experience; as well as all of the fruit of the contemplative life; as well as freedom from all of the sins/fetters/addictions.