On the assumption you mean 30 deg F and not 30 deg C (which would make leaving your banana outdoors impossible), I think a little more insulation round the pot and possibly over the banana plant itself is called for, although it's not entirely clear to me whether your sun porch is fully enclosed, or actually open to the elements.
I don't recommend you cut it back. Leave it as it is because, although the topgrowth may be damaged by cold weather, it will shoot from the roots again next year, provided the pot hasn't frozen during winter, and that topgrowth will help to protect the roots and growing point.
I suggest you insulate the pot - if you can, sit it in a box of some sort or a much larger pot, and pad round the pot inside the box with straw or bubble wrap or even horticultural fleece, anything that insulates. Crumpled newspaper would do, assuming your plant isn't fully exposed to rain or snow, being under the roof of the sun porch (hopefully its got a roof). Alternatively, you'll have to chance just wrapping the pot with fleece and bubble wrap over the top of the fleece, and a layer of mulch actually on top of the soil in the pot. In regard to horticultural fleece, wrap that round the topgrowth when the weather is very cold, but it must be removed once the cold weather warms up a bit, the fleece should not be left round the topgrowth all winter. If your sun porch is fully enclosed,you only need the fleece over the top at night when it's cold, remove it during the day.
In regard to water, the soil should be damp when you pack it up for winter; just check occasionally that it hasn't completely dried out to desert like conditions, and water sparingly if necessary. If your sun porch is fully enclosed, it may need watering more often than if its outside.