Stay as far away from the W58 tranny as possible, its a ticking time bomb. Very few have had that tranny with 500hp for more than a few thousand miles. its usually only good until about 450hp before it goes. If your going to spend the money on a swap, R154 should be good up to the 700's and the V16x (6 speed Supra tranny) is good as far as you can push the motor for the most part.it would be cheaper to do auto at that point, you'd need a valve body upgrade and transmission cooler, if you went manual you could swap in a W58, R154, V160 or V161 depending on how much further you are going.
How can you loose boost on a naturally aspirated car?? Boost is created by forced induction, ie turbo charger or supercharger. The IS300 does not have either. I think you received false information.also i saw somewhere where it said cars that are year 2000 + up are better auto than manual because it handles a little better , you wont miss shifts , no boost is lost in between the shifts , and you can build boost on the car but I dont know thats where i read it somewhere on a is300 video