The smallest cog that will physically fit on a rear wheel these days is an 11-tooth cog, which provides some "top end" improvement over a 12-tooth cog. If you want big changes though, you need to address the front end of the gearing -- i.e., bigger chainrings, not smaller cogs.

One other thing to consider -- as the tooth-gap of the front chainrings increase, shifting gets progressively worse. i.e., compact chainrings have a 16-tooth gap (34 to 50), which is "pushing it" in terms of the jump that the derailleur can handle. i.e., put a dramatically bigger chainring on and you might need a bigger "small chainring" as well which is going to affect your low-end gearing rather dramatically.

What sort of speeds are you maxing out at? Most people stop pedaling around 35, 40mph -- the power output required by you to push the speed higher as the speeds creep up into the 40mph range is "a lot".

If you want to get faster on downhills... get deep-section rims or work on your tuck. Aerodynamics are your friend at downhill speeds.