As far as the spool. definitely go for it. I have locked the front diff with putty and it worked great but then I changed to the spool. Man what a difference. I increased my acceleration out of the corner and decreased my lap times. It may make the car a little twitchy on corner exit but once you get used to it you will definitely love the improvement. Remember you are transferring the power quicker because of the reduction in rotating mass. The car will pull harder when you hit the gas.
Once you install the spool keep an eye on the axles and cups. You will experience more wear on these components but I think its worth it.
Oh and you will need the spool, gear, and the outdrive cups.