A shift kit is fitted to the valve body inside an automatic transmission to change the shifting times or adaption pressure between gear changes. So it can hold a gear for a longer period of time.
You shouldnt necessarily get one one. You could consider it if you been modifying your car already or plan to it along with some other modifications to your car.
$45.70 f**k knows.
As said it above, gives alot better stronger shifts at high revs, only thing is if you Granny it it's very uncomfortable for some people.
You can get the Corvette servo + Stage II shift kit fitted for around $350 in Bayswater Automotive.