Ian, have you looked at the Guy Croft website ? He's an ex F1 engineer that specialises in Fiat Twin Cams but he's very knowledgeable and happy (sometimes too happy ) to share it. I'd be surprised if he hasn't worked on Busso V6s before.
He got 300bhp from a 2.0 Rover KV6 (OK full race @ 10,500rpm but even so) and 2.0 Hot rod engines to 190 bhp at less than 8000 rpm with very fat torque curves.
NB If he thinks you're full of **** he'll probably tell you so!
I'll see what Guy Croft has to say then. Thanks for the info though.