There are batteries with horizontal plate orientation. Most I have seen are AGM. They use the weight of the stack to compress the glass mat to reduce internal resistance. Just remember the surface area of the plates (+) is going to be a major factor in determining the battery capacity. Orientation alone will have a little effect. Also horizontal plates do not provide good electrolyte mixing since gas bubbles can form on the plates and can produce "dry" pockets on the plates.
Eric.
