Hey Dub.
My car always seemed to run warm - 220 and I could keep the temps down ok by running the a/c.
First thing to always check on a C5 is that there is not a bag or debris blocking the radiator as our cars are bottom breathers and have a habit of picking up crap off the streets.
Mine was fine and it turned out that my coolant tank (surge tank) had a small crack in the nipple where the rubber tube attaches to. It was a small leak. I never saw coolant on the ground or leaking down the side of the tank.
However, I knew it was cracked due to the coolant getting darker and I could see just a bit of dried residue on the outside.
I replaced the tank and had the fans tuned to come on at 180 for stage 1 and 200 for stage 2. It's been great ever since.
Long story long, do you have any small cracks or pin holes in the system? Is the coolant still super clean or is it a bit brown like oxygen is getting in it?
Lastly, as another person interesting in getting twin turbos for my car, I'm interested to hear what other mods you are considering (may have to start a new thread). But my thoughts were for starters, new clutch, tunnel plate, DTE brace, and trans brace. Then down the line if needed, motor mounts, new bushings in the torque tube, hardened half shaft in the diff, and rebuilt transmission.
What have you thought of? My goal would be 500 rwhp so not quite as high as your 7-8 hundred.