in your case it doesn't depend on how much boost your motor can handle. your tranny will blow before your motor does, even with the lentech valvebody. i think 5psi-7psi on a good tune is doable on a stock atx with just a lentech valvebody.
Some where between 3-20psi depending on how good of a tuner you are.
I dont think any one has had a turbo Auto SPI long enough to say for sure how long the tranny will last. But if the ATX in the Sedan is the same as the Zx3, then ask those guy's how much power the ATX is good for.
when I called lentech before i bought my vb, i was told that just the new vb upped the power handling capacity of the atx by about 50hp to, i think it was, 250hp. I cannot remember what they said tq increased by