Had a recent us found a gallstone 8.6mm x 5.4mm. I'm symptomatic, I've read that post-choly, most patients still experience pain. Any truth to this?

Postop, not GB pain. Cholecystectomy is surgery, whether laparoscopic (belly button) or open (traditional). There will likely be post op pain (usually less and for a shorter period of time if laparoscopic). If your pain was gb pain (right upper quad/or mid below ribcage, maybe into back , often with vomiting, confirmed with us findings, e.g. Thickened gb wall, edema, fluid around gb, then surgery should resolve issue.
Yes. One of the risks of any surgery is chronic pain, but this is not the rule, but the exception. The vast majority of patients get relief from their symptoms after removal of the gallbladder with stones.
Yes. Just because you have pain and a gallstone, doesn't mean that the gallstone is necessarily causing your symptoms. This is why patients are told beforehand that there is no guarantee that surgery will resolve symptoms.