I've seen a Courtney Turbo'ed V6 Estate.....
suffice to say, that when i saw it, it was busy dumping it's coolant on the forecourt at our local sainsbury's petrol station.
which kind of put me off the idea.... 
Court
enay (when they were known as Courtenay Turbos) made some special turbo V6's for the RUC in Ireland, however they were only ever a low blow and produced around 250bhp, but iirc lots more torque. You used to be able to buy the full kit including 225 grooved discs (@ £200odd/ pair!!), think they stopped selling the full kit about 8-10 years ago.
To be honest if you want to go forced induction then just stick in a 4 pot 2ltr C20LET. This can be tuned for less to a 'fairly' reliable 280-290bhp
The ''blown' 2.8 V6 from the Vectra C and Insignia will be a nightmare due to it being full canbus. This is why there are no early half canbus Vectra C/ Signums with this engine conversion
Couple of years ago at Santa Pod there was an Omega with a 'Rover' V8 but it wasn't running well at all so no idea if they ever finished it