This is TamaDrummerBoyofDoom. I forgot my password, so I'm going to use Tamsabian from now on.
Double bassing is only as usefull as you make it. I am at 260bpm right now. In common time, I can play 16th notes with each individual foot. I other words with just my right foot i can play 16th notes (1e+a2e+a3e+a4e+a), and with just my left foot I can play 16th notes (1e+a2e+a3e+a4e+a). Combine them and you get 32nd notes in common time (260bpm).
How? Well there are three factors. First is your style of pedaling, second is the setup of the pedal, and third is the setup of the throne. I use a Tama Iron Cobra Junior (not the Iron cobra regular, mine has different beaters and reads TAMA on the pedals. Costs less too...haha) If you get a pedal, get one that will not burn a hole in you wallet, but will also allow for adjustabilty of the position of the beaters.
Setup of the throne: First, raise your throne so that your thighs are parallel to the pedals. Then raise it an additional 2 inches. Your legs should be pointing downward, but not hanging. Then place a piece of 1/4 of an inch of ply wood underneath the back legs of the throne so you back with be tucked in. This prevents you from leaning back, and tiring out you lower back.
Setup of the pedal: Now the pedal modifications. Lower the beaters towards the bass drum head so that the beaters are about 4 inches from the bass drum head. This will not reduce the volume when you pedal the bass drum.
How to pedal: Now that you legs are on the throne resting, pedal using only your foot. Simply tap your foot on the pedal. You'll notice that you use only your calf, rather than your whole leg. You never want to generate your power from your hips, lower back, or thighs, because they will all get sore and tire easily. Remember to make full strokes, returning back to the pedal's original position before making another stroke, and always keep your foot on the pedal. Note that you can only get above 240 bpm by this method.