I don't think any amount of strengthening of the transmission would allow you to put that much power/torque through it in the first place. The transmission would explode before the engine had the chance to shred the tyres apart from one WOT.
You say it's hard operating the clutch and steering a normal Diablo, can you imagine what it would be feel like to drive a Diablo with a 400% increase in power! And NOS!
800bhp sounds more reasonable and you'd think the Diablo would be capable of dealing with this. Would the owner be capable of deal with it though?