First of all, it's not the psi that matters, it's the torque that kills driveline components. Not only that but it also depends on how the person drives, supporting mods (cooling,
vb upgrade, etc.) as well as how the power is delivered (instant power from a roots type blower vs a big laggy turbo). There are way too many variables here to decide how much a tranny will handle and at which psi level it will blow. Sam simply played it safe and installed a 6PSI pulley. Not only for the tranny but probably also because Shumby will be driving nonstop for a full day with a car that now has 130+ more whp. Want more power? swap the pullies. It's a 20min job.
Also, the 5AT and 7AT trannies should be able to handle more than 400whp WITH supporting mods no problem. I don't know if you guys visit the G forums but there's been numerous G35 HR's and G37's with TT kits that are auto pushing 450whp and more. If it wasn't safe for those guys (some of which have been daily driving their cars for YEARS now), you'd be hearing tranny horror stories all over the forums. Not to mention Sam would not be pushing the AT's anywhere near 400whp.