Turbobuick guys have been running into the 10's for decades on completely stock frames or stock frames with minimal bracing.
G-bodies will always be a good starting point. They are reasonably priced and have decent aftermarket support. The weak areas are easy to address (frame, transmission, rear end).
The C5 will have its own issues.......initial cost of the car is higher, if auto....a reliable 4l60e is costly....and there is not much room in the engine bay.
That is why so many Turbo Regals have warp and twist damage to their bodies and frames. Doors that no longer close right, winkled rear quarters blown rear windshields, twisted rear LCA mounts, etc. Regals, except the GNX had the worst body and frame setups with deleted bracing and body mounts, a pretty sick joke from GM.
Except aftermarket support is falling off and remaining new replacement parts are sub par. As I said, they were never designed for 400 hp, even 300 is a serious strain on them. By the time you reinforce the frame and body along with replacing the trans and rear with stonger units, the build is already no longer low cost. Its like driving a Model T in the 60's.
Still has a far better frame, far far better suspension stock, ABS brakes, airbags, better rear, better weight distribution, modern A/C, no 40 years worth of rust damage to weld, no 40 years of PO jerry rigging, everything is better. There is nothing low cost making a G body match a C5, or even a C4. Even a 4th gen Camaro is a far better starting point for modern performance.