Unless you're running a 32-tooth chainring or smaller on your DH rig, you need an 11-tooth cog like a fish needs e-mail. The tighter spacing will yield crisper shifting, which will play a much bigger role in your race run than that higher top gear. SRAM's PG-950 12-26 cassette is a great option, especially at this price.
What SRAM Says:
When it's time to replace your 9-speed cassette, check out SRAM's PG-950 cassette. It delivers solid shifting and durability ride in and ride out for your 9-speed rig.