I went 6 speed for a very good reason and for once did some research on the subject !
I have a car that was dyno proven to run 580BHP at the fly, but during the last AUTO mapping session I had at 380-400 BHP fly the box started slipping, this was due I would have thought to constant 500+ use over time and was to be expected I suppose.
I brought an Auto on purpose not because I could not afford a six speed as I drive in traffic in London a lot and the Auto box is a dream of a box for this type of driving, however it is no point having a 500+ bhp car and not be able to use it so I looked at the TH400 box and a BL box, it was a close call (TH400 too clunky and a bit aggresive) and I almost went BL but the 6 speed came up and was fitted and I have been very happy with that decision so far, all be it traffic is still a PITA