don't buy the locker. (most lockers are a PITA to switch anyway). The only time you would want your posi locked is at the 1/4 track, and that is it. If you go around a corner, and the wheels are locked up, then one cannot turn faster than the other, and you will wear your tires fast and get the screaching sound. as for a FMS posi taking 1-2sec, divide that number by about 1000. It takes generally a few milliseconds for a differential to react to slippage.
I had an auburn posi in my 8.8 when i had my mustang, and loved it. the FMS one is good and will hold enough power until you need a stronger one due to a huge blower and whatnot, but by then, you'll need to change to 33spline axles anyway.