Shift Kit?

i have a 98 trans am (automatic) with cat-back exhaust hypertech power programming with 160 powerstat and just had 3.73 gears installed. Im curious to know if 1. should i get a shift kit? 2. does it let you put the shifter in any gear at any time, like downshifting to 1st when your going 25 mph? and 3. what other kind of improvements will i get?