The Dwarven Gunner disciplines behave identically for Bor pistols. They have no direct effect on cannons or bombards as these load and operate in a completely different fashion.
While it is not as useful for a Gunner with pistols, it does allow one of the two pistols to load as rapidly as a rifle. It would not be a stretch for a Games Master to allow a Dwarven Gunner of higher than Rank 10 to allow both pistols to be reloaded with this discipline. (I would allow it.)
While there is no direct game benefit, superior and magical versions of cannons and bombards could have increased resistance to external damage, require less powder per shot or have special effects when firing. The options are limitless.
Boom Powder is as vulnerable to liquid contact as black powder with one exception. It is not ruined or damaged in any way when put in contact with alcoholic beverages. The Borese worked long and hard to perfect that trick, as they were tired of drunk Gunners ruining their stock by mixing up flagons...
There you go!