Is it pretty much de rigeur that a keel stepped mast will have some leakage through the inside of the mast into the bilge in a good storm? Mine does, it's very minor and not enough to even empty with the bilge pump. Seems like the masthead fitting by necessity has alot of openings that blowing rain can find there way into.
If you assemble from scratch you can avoid it.
We never had any wires run down through the inside of the section so filling it with blown foam at deck level, whilst ensuring that foam is level with a rope outlet (so water coming in higher up can drain out of that) reduces the water that gets below.
Having said all that we've have had trouble on one boat sealing outside twix mast and deck! Thats boats for you.