I've used a Flextight 646 a bit and not had many problems with grain, but I do use Xtol which is quite a fine grain developer. My main film these days is Acros, which is pretty much devoid of any grain, but I'd imagine you'll do well with Efke 25 as I used to shoot quite a lot of the old Agfa films - which I believe share some similarities - and they scan beautifully.
I wasn't clear from your reply whether you've tried scanning as a positive and then inverting in Photoshop - but that's the single thing that's made the most difference in my film scans. Although I have no rational explanation for the difference!