Adjustment to road bike brakes for high grade downhill
Have a look at installing Cantilever brakes- larger pads so you can retain the same stopping power and increase life, at the expense of a few grams of weight.  Cantilever brakes will also help in the wet due large pad (more surface area). These are the preferred brakes for tandems and tourers - you may have trouble getting decent quality ones these days. As I understand, V brakes are not compatible with road bikes without adapters and are not always as successful as you would think. 

Discs would be the ultimate wet weather improvement and provide the stopping power you want, but I would caution against hydraulics for such big downhills. They have been known overheat and boil the fluid - you get instant total failure when it happens. Apart form this, you need new forks and wheel (minimum of a hub), and probably new levers, so the cost may be prohibitive.