So far, I have found no difference in performance of the BMW AGMs (made by Varta) and the aftermarket Varta AGMs private labeled under various names. Other than case color, they seem to be identical in construction. Both seem to last a very long time.
I have definitely seen differences in oem/oes BMW parts, but the performance and longevity differences are directly attributed to different manufacturing plants and countires. For example I had terrible luck with Behr aftermarket expansion tanks (made in China) constantly showing "low coolant" unless overfilled. After three tries I bought an OEM Behr made in Germany, no issues. Hengst is now offering engine air filters made in China for the aftermarket....but their BMW labeled ones are still made in Germany.
But the Varta batteries are all from Germany and presumably from the same plant.
When I worked for Duracell decades ago, we asked if the private labeled batteries Duracell made for Costco, Safeway, True Value were any different, and Duracell said no, it would actually cost them more to make special variants just for private label. Most retailers discovered how cheap it was to source in China, Indonesia, and Korea, so Duracell only provides to Costco anymore.