Lol, i was just taking the piss with the 2.75" thing. Thats very interesting about the dual water to air with separate pumps. Apparently it had air-con too!
Getting 800hp ot of a 500hp turbo engine isnt really that astonishing - I'd imagine that's fairly restrained. Can force as much air and fuel in as you want. Especially the jap turbo engines of the last 20- 30 years, can usually at least double the output on stock internals.
The Gt3, i imagine could have had as much as they wanted - it was a bit of a controversy how much boost they were running vs how much they said they would be running.