I think Crawford was running over 400 WHP on an AT BRZ without many issues from the Tranny...the motor was another story lol. There are a number of people with ATs hitting 300+ WHP. This AT is pretty strong, so probably won't need to mod it much. P&L hit 480+ WHP on stock auto tranny.
But heres what you can do to it
http://www.ft86club.com/forums/showthread.php?t=18868
What Lunatic said is a must for forced induction ATs.
If you plan on reaching 300+WHP I assume you'll go Turbo route as its much cheaper than supercharger. In that case, maybe just get a 3" Catback right off the back instead of Isis's 2.75"?
If you'll get Blitz Intake (its really nice I hear) check if it needs to have the MAF scaled if it does, you better get a tune with it, it'll drive SO much better. If you opt for the Openflash route with tuning, check if Blitz includes an already Scaled MAF so you can just plug it in yourself.
And yeah...So much information out there as Tcoat said go over to the tech threads haha, and use google to search this forum...its a lot better than the forum's search function lol
Good Luck!