Based on recent research on the topic, with Domanskis and Stokes as the main contributors, here is my two cents. Please note I am *not* quoting them on anything, as information often times gets muddled and mistakes happen. General information only.
Stokes used to use Allergen's expander. Allergen stopped making/marketing their expander implants. Stokes was looking for a replacement as recent as this last month at a convention in LA. She kind of has a replacement.
The old models, her office reported one fill up to 14,000cc and one pair installed and still OK after 12 years.
The best option available (now) to her is 1000cc rated. She feels confident it can go 4,000-6,000.
Domanskis has used PMT's expanders, which have a remote fill port, for many years. They still make custom implants, rated at 2,000cc each, and the devices had a retail cost of 2,400$ each. Based on memory, he had confirmed fills up to 11,000cc.
The Mentor Mod+ 800cc devices that many overfilled women have are indeed resilient, but they do fail. We have had them fail in as little as a few months. Overfilling to 4x the rated capacity on any device will cause complications. Couple that with an active lifestyle, and you get expensive failures.
Expanders tend to make a bit of snoopy-boob, even when sub-luminal, no matter who installs them. This may just be a side effect of the sizes involved, as the implant eventually comes out from under the pectoral muscle. Kind of like how most implants eventually (usually) end up with a nipple placement above center line. Comes with the territory.