I made it back (https://eatlikeushi.posthaven.com/imren-grill-berlin-07-2018), this time to the original branch and the one most glorified on the Berlin Food Stories website.
I wasn't as enamoured as the first visit, but it was still very good. The doner im brot was €4 - not sure if that's the branch or just the prices have gone up in the past year.