Well my trans is built as far as anyone can go,I even cryogenicly treated my gears,r&p and case.
I had maybe 325 horsepower and could break it whenever I wanted.I had to drive it easy but that hardly ever happened.Everyone here can attest to that.
Id say if you built one with the Aluminum case(very very rare) all weddle gears including the new ring and pinion(something I never did),super diff with billet side cover and gusseted the case it would probably hold 400 hp but you could break it if you got stupid.
The Weddle r&p just came out about 2 years ago I think and is $700 by itself.