The lower stock arms do flex even with a stock rear sway bar, in fact the stock rear sway bars cause some binding but not on the scale that boxed lower arms and poly bushings cause. Overall the stock rear sway bars are a rather poor design and why the aftermarket axle mount rear sway bars are a huge improvement over the stock bar. Probably one of the biggest rear suspension upgrade you can perform. I have seen turbo Buicks snap the stock rear sway bars like a twig because they binded up to the point of failure during launch.
Currie and Chassisworks also make lower rear arms with roto joints.
The stronger and more rigid the rear control arms are, the more stress you induce to the mounting points on the frame. Many turbo Buicks have twisted or sheared their rear lower control arm frame mounts. These cars were designed first and foremost to be grandpa cars, the performance models were marketing afterthoughts and suffer from several design limitations because of their main intended role as grandpa cars. Overall, the 4 link suspension is a very outdated design with major flaws that are very difficult to fix without making other aspects of it worse. Fix the wheel hop you make binding worse and vice versa. The 4 link is also a poor match to a front double wishbone suspension, just lots of major flaws. G bodies are not corvettes and never will be.
