I installed a Rear lower tie bar, a rear upper tie bar on my EG, and i was planning to buy & install a C pillar bar. If i intend to add a C pillar bar for the rear, will it give any extra difference on stiffening the rear?

 

wait.

lower tie bar - ok
upper tie bar? you mean shock tower bar? - ok
C-pillar bar - nearly useless
SWAY bar - one of the best things you can do to your car. depending on the car an ASR brace and a CTR 22mm swaybar usually does perfectly.

forgot to mention if you do end up with ASR brace it renders a separate lower tie bar useless because it functions as a tie bar as well.

 

C pillar bar without triangulation may not do anything, and even with triagulation may not add significant stiffness. Perhaps bling, or clothes rack. I tried in Integra and cannot notice much/any.