If you use sweet potatoes in an extract batch, you will need to mash the potatoes at the correct temp to convert the starches. If you search this forum for "sweet potato" you'll find a paper giving the diastatic power of sweet potatoes. My batch turned out tasty, but still too starchy. (I used a crockpot to cook the shredded potatoes, 5lbs worth IIRC). I'm guessing if I would have held it at mash temperatures it would have converted better.
You may just want to buy some amylase enzyme to make it easy on yourself.
edit: here's the paper giving SP info: http://www.jbc.org/content/44/1/19.full.pdf