I agree with the suggestion to go to a size that allows the full use of materials. Remember, the size is the outside of the building, what is known as the "building line", never inside dimensions.
Unless you are working on motorcycles, smart cars, or Trabants, you should consider making the door larger. 8 ft wide is a squeeze for most vehicles, and full size trucks would be tight, and duallies won't fit. I would not want anything less than a 10x10.
Reinventing the wheel, with something unusual such as the corner posts you suggest, is probably a waste of time and money. There are proven construction techniques for both steel and wood, and you probably should stick to them. For a regular steel building that size, it will be columns and trusses of C channel, and it will be quite strong. For wood, the corners will be doubled or tripled (or more) two bys, either 2x4 or 2x6. You also might consider the steel structures that are similar to the carport structures, but usually feature more steel and strength, especially in the roof. They are built with 2x2 thinwall tubing.
Charles