Do replacement batteries work with battery health feature now?

You’re right, the feedback is mixed, and it’s because iFixit batteries are not 100% reliable on this yet—whether or not the battery health feature will work depends on your exact model iPhone, which iOS version you have installed, and the battery’s internal firmware version. We’ve developed a long-term solution that should work for all batteries on all past (and future) versions of iOS, and it’s making its way out over the next month or two. So if you want the battery health feature and you have the luxury of waiting a bit, you may want to check back in February or March.

To your second question: the phone is only throttled if the battery is unable to supply the needed voltage. So yeah, running benchmarking software like Geekbench is a great way to verify that everything is ship shape.

Hope this helps!