I wouldn't worry about downhill performance so much. Folk of our ability are much better off getting into a nice comfy yet aerodynamic position and worry about spotting hazards and braking for corners!
We can gain far more speed by being smooth and tidy through the bends at high speed rather than worry about spinning out a 50x12. Remember every increase in speed requires an increase of the cube of power to overcome air resistance. Trying to accelerate from 30 to 33 mph by pedalling costs us a lot of energy - much easier to get on the drops, cover the brakes, relax and concentrate on the bends.
Like Banjo said If you changed from your (probably if you have a standard road bike) 12-25 cassette of sprockets to a 14-27, you'd probably not notice the difference in top speed, but you would appreciate the low 27tooth gear for climbing hills and the nice close gear ratios which will enable you to keep to a constant cadence - which is much easier to achieve and more comfortable than a constant power output.
Oh, I live in hilly South Wales btw - where we have the Dragon Ride like.
Another hill tip is to always keep pedalling over the top of the hill and DOWN THE OTHER SIDE! You don't have to put much effort in - just keep the legs turning over and flicking up through the gears as you spin them out. When you get to the bottom of the hill you'll be moving much faster than if you merely freewheeled down and your legs will not be as stiff - you'll also not have to accelerate to your regular cruising speed - just gradually change down until you can maintain a gear.
The alternative is to freewheel from the top after caning your legs on the climb, allowing lactic acid to remain in your muscles. They - and you - cool down rapidly on the descent. You get to the base of the hill and slow down. You think that you better start pedalling again and then realise you have to accelerate to your regular cruising speed with cold, stiff legs that are powered with muscles that remain full of lactic acid.
Try it - it works like a bloody charm!