Thank you for the welcome.
Callaway didn't really keep very accurate records on how many they sold as they offered the Callaway option as both an addition to the standard GTV6 purchased at the dealership and also a primitive "by the 80's" after-market option in which they would sell a GTV6 owner the system and a wing (plus wheels if they wanted to go that far) and then a tech would install the twin-turbo set-up at their local dealership. Whilst the 30-36 number is close, I suspect there are/were a few extra systems bouncing around after Callaway ceased working with Alfa Romeo and BMW and went strictly to doing the twin-turbos on the American Corvette.
A unrestored Callaway ought be able to pump out everybit of 260 horses. I had this motorcar ground up restored and a fellow named Gregg Armstrong of Historic Race Engineering in Costa Mesa California who designed the Twin-Turbo system for Lambo's Diablo redid eveything. In some cases he suggested updating to modern standards certain things to make the Callaway more reliable such as a 40% larger engine coolant system and radiator, bigger fans, larger brakes, modern fuel management, lightened gears, and even modern climate control and power steering. This is really the tip of what he did as I honestly can't remember all the little bits he done without inspecting my records. But it does kick out 329 horsepower now which for an 80's V6 isn't bad at all. Especailly when you consider the meager horsepower of other contemporaries such as the Ferrari 308 and normally aspirated 911 Carrera of the same time period.
It isn't a monster car by modern standards at all, but it is loads of fun to belt around in.
I just hope I can get it through MOT.