If you're spinning out at 28-30mph going downhill with your current gearing you probably need to work on spinning at a higher cadence. Then again, how steep a grade in how much wind are we talking? There are some descents around here that I get nailed with so much headwind I'm slower than on the flats.

One cassette isn't going to make you faster than another, per se. The thing you need to think about are how close the gears are together. With a narrower range you wont make such big leaps between shifts. Now, at a given grade with a given amount of headwind you might find that at your preferred cadence a difference of one or two teeth will make all the difference. With a larger gap between cogs you might find that one gear is slightly too low and another slightly too high.

I'd try a few cheaper cassettes before you really nail down which one is best for you. If you're a heavier person on a heavier bike carrying some extra gear on top of it your needs are going to be different than somebody with a strict roadie setup. Try a wider range cassette to start and work your way down from there.