See: BoA (Not an exceptional singer like you two suggest needed to become a solo act), Hyuna, Lee Hyori (this is even more relevant as Lee Hyori was in Fin.K.L and she wasn't even the standout of them), Jay Park, J.Y.P, Simon D, literally everyone.
You should NOT be judging idols on how well they sing, they're idols after all. Soyeon is in no way T-ara's best performer, she has her moments, like when she's given vocal lines and allowed to demonstrate her capability, but she is not the best performer. "a poor performer(in a group not known for being particulary good at either)." that line was so confusing, but I'm going to take it at face value. T-ara not known for being preformers? You lost me, T-ara is literally all performances, for the EXACT reason you mentioned: not one of them has standout vocals. The reason they made it this far hasn't been on any members efforts alone. The reason T-ara got popular was was SoJiEunMin, Soyeon with her comedy and witty attitude on Oh! My school. Jiyeon for being the face of the group and being compared to Kim Tae Hee, Eunjung for showcasing her abilities in Dream High, and Hyomin for showing her personality and worth on Invincible Youth. Not to mention those 4 were paired with SuperNova, the literal reason they got famous. Eunjung and Hyomin aren't competing against eachother, Eunjung probably DOES want to go solo as a singer, and they all want to remain T-ara as well, however her agency (CCM) does not see it fit for her because their prospects of getting money out of the whole event is low. Eunjung as a singer being better than Hyomin is questionable as well, she's not the type of person to carry a song and control her voice, she's more the belter of the group. Look at any performance with the two of them, Hyomin usually does most of the singing when they are together. Eunjung defeats Hyomin in areas of old style singing and Trot, no doubt about that. As long as they don't Autotune Hyomin to hell, I'm fine with Brave Brothers, pls no sistar rehash song though.
Y'all should check out the Nine Muses idol documentary, btw. It gives you insight into this industry, and makes you see what idols actually go through, and have to live through. Kind of sad, esp thinking about how CCM treats them.
(I was upset when watching Hello Baby and seeing how small their vocal practice room was T_T)