It locks up after 8 mph probably because if it locked up at any lower of a speed, the engine would be too close to stalling.
Also, the ZF 6HPxx (and newer) transmissions behave the same way. My 2011 335i will lock up in first gear (probably around 10 mph, but it depends on the load). This transmission has been used by BMW since at least 2006. A derivative of it is also used in the Ford Mustang now.
I should try a car with one of those transmissions, then. They sound quite interesting. While I have driven quite a lot of cars, I have never dealt with such a transmission, but it sounds both nice and efficient. How's the comfort tradeoff? I believe the lock-up was initially avoided due to comfort, as people liked the "gooey" transition between gears.
I'll keep that transmission in mind next time I go shopping.
Also, the ZF 6HPxx (and newer) transmissions behave the same way. My 2011 335i will lock up in first gear (probably around 10 mph, but it depends on the load). This transmission has been used by BMW since at least 2006. A derivative of it is also used in the Ford Mustang now.