That is the "drum" people refer to as being the weak link. It is the forward clutch housing, and can break under certain circumstances.
As someone who does not manufacter this part, I think with an Art Carr 9in converter you will be fine to make more power than your headgaskets can.
A properly built trans, that does not give you whiplash when it shifts should live a long service life.
Now, that being said, if you were running even a mild shifting trans with a stock 12in or restalled 12in converter, I would say you need that billet shaft for added safety. Given the converter you have chosen you should be good.
To answer your original question... I am not fimilar with your builder or parts they used. But, I would say if built correctly, 400-500hp should be attainable.
Hope this helps...
Brian