if your car is still understeering alot even with all your mods, id have to say its your driving. all front wheel drive cars will suffer with power understeer if your to aggressive on the loud pedal during cornering.
i had eibach ARBs on mine. i found that with the rearbar on its stiffest setting and the front on its softest that understeer was at its lowest. never really had an understeer problem
if i was to do it again i would fit an even stiffer rear bar because i like a loose back end, with a more oversteer bias.
you could look into a BSH 27mm rear ARB and flog your eibach rear bar. however you start to lose independance of the suspension by having such thick bars on the car.
a better alternative by a long way would be the TT lower control arms, the extra 0.5o neg camber will massively increase cornering grip. thats only about £400 fitted