the six speed isn't a five with an extra gear tagged on. The final drive in top is about the same in both so the difference is that the intermediate ratios are closer together than in the five speed. This is actually particularly useful in the TS which is a peakier engine because it means you don't fall so far down the power curve when you change up a gear so it should really improve driveability and performance when accelerating. This isn't so much of an issue with torquier engines such as the V6 where the power band is much broader, which always leads me to think that offering the six speed on the V6s was always just a marketing gimmick - you end up doing more gearchanges when you don't need to!
So in theory it is a good idea for the TS but don't know if it is possible. You would also need the cable operated linkage from a GTV V6 - assuming the 166 is different.