1) Twin Turbo will be the only option to reach your power goals, the Stillen SC will fall around 450whp
2) There are guys running 500whp with just a valve body upgrade, but it really comes down to being a ticking time bomb with that. I am sure it can last, but the harder you push it the weaker it will get.
3) Other supporting mods for just the tranny will be a good tune that will adjust the transmission pressure lines to help hold the power.
Now back to point 2, the main weak point in the transmission so far seems to be going into boost in gears 5,6,7 as they are the weakest. Though a hard launch in 1 will definitely cause some major wear, but it seems the higher gears are not able to handle the TQ levels of any boost application (TT/T/SC). The transmission is only rated for about 295lb of TQ so adding boost you are adding easily 100lb more.
I know you have been reading, but in case you missed this thread it is full of info for the transmission & boost:
Forced Induction & 7AT 370z Info