Weakest Link

[Hammer]

The T45 and the rear end are not weak, they just weren't designed for applications with high-torque numbers...

My stock T45 lasted about a year with about 450 hp at the crank. I'm currently looking at a Tremec TKO for replacement.
The stock rear end could use some work right about now too, so I expect to do some work back there also in the spring. (Bearings, Shims and seals)

As far as blowers, the amount of boost you can run on a near stock setup is usually determined by what type of blower you are looking for.
With a roots type, I would stay around 6 psi.
A centrifugal blower, about 8 psi.

I would advise some engine work before going any higher than that.

As in any blown application, watch your compression ratio and be safe out there...