My recommendation is that you use the lightweight graphite composite chassis (which comes with the TA-04R chassis kit). If your kit already has it, then you don't really need to spend more on a lower chassis upgrade. The lightweight chassis can accept saddles packs - just remember to line the battery compartment with some insulation (such as some scrap polycarbonate) as the chassis conducts electricity and may short out any exposed battery packs.
The benefit of the LW chassis is that it should be stronger than the full CG chassis and longer lasting. Weight difference is negligible (I compared my TA-04R against my 414m2 and they weighed about the same).
The TA-04R works damn well straight out of the box. However, if you must upgrade, then I suggest the following upgrades are your best bet for improving the performance of the car:
(1) Hard suspension arms;
(2) Racing body mounts (this will improve the sit of your body on the chassis);
(3) centre one way unit (you can either get the plastic set or get the aluminium one way pulley and an aluminium centre pulley combination);
(4) front one way unit (this is driver preference and dependent on the type of track you drive on); and
(5) Aluminium ball diff pulleys (lasts longer and is smoother than the standard plastic pulleys).
To me, everything else is cosmetic surgery (but they can sure make your car look really nice). Having said that, my TA-04R has had full cosmetic surgery and is effectively a TA-04R Blue Edition (running centre and front one ways).