the main problem of Automatic trannies is slipping.
If it slips, it burns.
Since you have a higher T/C stall rate, and an improved pressure line in your transmission, the clutches will grip more then, reducing slip.
What kills a transmission is kicking it when it is shifting, or stalling the trans on dragstrip (holding brake and gas).
and the main reason why A/T transmissions dies, is heat. Hot hydrolic oil = no oiling properties. Your synthetic oil will also help the transmission to keep up the good work even under very hot and hard conditions.
I have no doubt the A-340E can handle a lot of power. But the clutches and pressure might be the problem. High pressure, hard clutches = harder shifts, good for drag strip but not comfortable for the everyday driver. I really don,t think someone will strip a gear below 500WHP.
And you're right, the heavier the car, the more the trans work to move the car.
Also, i'm not an exepert in transsmissions, but it might not be very good for the Torque converter to lock-up when you're applying 500 horsepowers on the transmission? It might be the reason why it breaks? Who knows. I'm just like you and want to know the most possible about those cars.
But i know that Dragme on cressida forums run in the 10 seconds with a built A-340E without any problems. (trans built by MV )