Wagner, if you practice AYP very faithfully, you will attain very great siddhis very quickly...and I can guarantee it. You will become kinder, more relaxed, and more patient. And you will eventually attain the magical power of not fighting with the world. Which soon leads to the highest of all siddhis: happiness.
Re: the sutras, I couldn't agree more with your recco*, Frank (Yogani has it on his book list, too). I've fallen head over heels in love with it. It's expensive at amazon, but cheaper at half.com. It's a must-buy IMO. I expected it to be marginally better than other translations, but there's no comparison.
*The Yoga Philosophy Of Patanjali - by Samkha-Yogacharya Swami Hariharananda Aranya