So in the turbo instance, before all else, you NEED to put an LSD in it
There is no point spending all that money, and spinning it all away through one wheel. As mentioned I don't guarantee the figures from GMAuthority, but a twin turbo is almost certainly going to at some point cause a rapid, unscheduled, disassembly of the box.
V8 swapping is an option, but again you'll be up for big coin, and in essence will always be a "hack" job (Not that is necessarily a bad thing, but it isn't an easy process) Places like GC cars Nerang offer the roll in roll out service, but they are minimum 15K
I think in some instances they don't even change the tail shaft to a V8 one (But maybe it was an option not taken up) but if thats the main service, instant no go from me, and its the same with the brakes. Don't know if they upgrade the brakes too
The final one you'll be potentially in the cross hairs for, is engineering compliance (Which will not be cheap) and the likely hammer blow, insurance.
Assuming you are younger, most insurance companies will here "Twin Turbo Kit", "Commodore" and under 25 as an example and will likely give you their "Get lost price" (Not saying thats your case, but be prepared for it)
As per your Bio, leave the Calais alone. Treat it well, learn all the ins and outs, maintain it and keep it as a comfortable daily drive. And save for an SS and avoid the hassle.