DO NOT CONVERT TO A TT SET-UP IT WILL KILL HP ON YOUR Z31 AND LIMIT YOUR CARS POTENTIAL. Single Turbo set ups are more efficient. Single Turbos flow more efficiently and are mechanically much simpler. More is not better in the world of Turbos please do not ruin your Z31 by wasting your money on a TT set up. Look at all the grand national cars (HIGH HIGH HP VEHICLES) they all are single turbos also if you look at alot of high power cars that were originally TT you will notice that they inevitably convert to a single turbo design. TTs will only give you off boost response they will never get you anywhere. Ball Bearing Turbos are nothing better then a normal Turbo in the end on a Z31 they are a waste of money Turbos spool faster becasue of lighter internals NOT BECAUSE OF BALL BEARINGS. If you want some ideas on Turbos look to Jim Wolfe Technologies
http://www.jimwolftechnology.com/ they have been working with Z31s for years and they really know their Zs.
The single turbo is much better keep the set up on the Z31 just upgrade your turbo.