TIR was a huge advance over the old hat switch or number pad...
TIR2 added a bit more sensitivity and responsiveness, and was more able to cope with background lighting (but still a bit poor IMO).
TIR3 was an incremental update to TIR2, and OOTB wasn't much to write home about...
...but it did have an optional 'vector' expansion, which extended it to 6dof, with more freedom of movement while retaining sensitivity to head rotation, and for software that supported it (not much at release
the additional pan/zoom options.
TIR4 is just a repackaged TIR3/vector, with slightly updated electronics, and TIR5 - I guess is just more of the same...
Once it works properly then there isn't much more to do than make it work slightly faster, with slightly less sensitivity to other lights - which is probably the worst problem with it, if your environment is poor (or GF won't live in the dark with the curtains closed so you can drive your tank/sub/airplane - some of them are difficult like that
)