First off, get some gears. One of THE best mods for your stang. 3.73's, 4.10's or 4.30's should be the only question you have. Do a search and ask some other stangers, if you know any with gears, to take a ride or drive thier cars.
As far as the predator goes, is it worth it. I think it is, others will say to wait untill you do more mods. I like my predator. A little on the SOTP meter, better throttle response, it adjusted my speedo for my gears, turned off the dash light for the O/R prochamber and it does other stuff that I haven't played with much. Adjusting A/F is best left alone unless you have a dyno tuner with you. You can adjust the timing and a few other things without doing much potential damage to your car. The instruction booklet is detailed and the menu screen is easy to navigate. And yes you have to run higher grade gas, unless you install the predator performance tune and then pull timing, which doesn't really make sense to me, but whatever. I think that covers your questions. Think of more and keep asking. I don't have or know much about Xcals but they seem popular also.