Hotchkis tvs.

In a driver car (not a drag car) would frame connectors and reinforcing the frame stiffens improve handling and cornering? Or is that just a straight line upgrade?
Depends on the car and what you mean by handling. Cornering on smooth pavement with run of the mill all season radials? no.
Driving on bumpy dirt roads? maybe a little.
Again some depends on the specific car, how well it was assembled, and current condition.
The rest is how much load and impact is going to be applied through the wheels.

Also can sway bar tabs be added on to lcas that dont have them on? Seems like you could just weld them on yourself. Id like to add a swaybar eventually but my lca dont have the tabs.
Yes, but positioning is critical.