Finetuning questions - do I need a high-stall torque converter ?

^^^ Good statements and not arguing. There are multiple things to consider, like torque converter spin at lower RPM's, which effects fuel economy and transmission fluid heating. And not just any unit will work this way; your statement about 'a custom quality unit' has to be true. Only certain brands fit that description; not all torque converters are equal. And even the good company's information needs to be taken with a grain of salt; they are advertising. (Basically, I don't necessarily believe every little thing that even the best TC companies say....)

IMO, the OP needs to balance best ET versus the time he spends doing other things with the car. He is in an urban area in Germany, and unless you are on the autobahn, average driving speeds tend to be slower than in the USA, even in US cities. Then call a company that makes 'a custom quality unit' and describe his use and speeds and let them make a recommendation. Some recommendations to him on the best companies for torque converters seems to be useful.