the problem is that you say your car has no rear sway bar. i cant remember if ej8's came with a rear sway bar or not.

but jsut adding a smaller stock one from an em1 or ek4 will make some noticeable difference and i think they are 14mm.

if you jsut want a slight improvement over what you have now go this option, its cheaper all you need to source are endlinks, swaybar, d brackets and bushes and bolt it on. oh LCA's if you dont have provisions for endlink bolts.

if you want a nice noticeable performance increase go the ASR an 22mm ITR rear sway bar.

this is a lot more expensive but well worth the gains and it seems like its what you are after performance wise.

you will need ASR brace, 22mm itr swaybar, d brackets, bushes and endlinks. great upgrade though