The core doesn't really matter. It's how it's put together on the inside.
The big not-so-secret secret of torque converters is only the OEMs have the money to develop them. That's why they use essentially the same basic design for decades at a time. The aftermarket just tunes them by tweaking the stator and turbine combinations using off-the-shelf components. So if they take a lo-po core, cut it open, and put high-po guts in it, it's going to be fine. Because there's really no such thing as a low-performance converter. Just a converter with the wrong calibration for your combination.
And every vendor has bad reviews. Nobody's perfect but everybody screams to the heavens when they're wronged or have a less than good experience, and most of the time they're blaming the vendor for something they screwed up. It used to be those voices wouldn't be heard much because they were rare, but the internet gives every nutcase a megaphone.
As for the price, how much is your time worth? You can spend weeks yanking that transmission over and over experimenting with various used converters; or call up a reputable torque converter builder, tell them what you have, and they'll send you something set up for it and it'll be right the first time. They're usually confident enough in their work that if it's not right, you can send it back and they'll tweak it for free until it is right.