You may improve a safety system, but any 5-point harness without pre-tensioners would be removal, not improvement of a safety system.
That assumes your car has those, lots of cars don't. The W126 Mercedes was the first car on which they became standard... in the long-wheelbase editions, which means my 1982 doesn't have them. But lots of newer cars don't have them either, like most cars of the 1990s.
A 5-point system helps even without a cage.
Not really. The seat mounts can't handle the load, and the original mounts aren't in the right place. The racing harness is designed to keep you in place in a car with proper crash protection around you. In the 1960s race cars still had wacky little half-seats, around then they started to become part of the crash protection and that's when race cars got harnesses.
A 5-point system without a cage is usually mounted to the seat, and usually reduces the safety of the vehicle as a result. You can get a cute little half-cage or even a roll bar with a proper place to mount the straps, so you don't need a full cage, but you need to add something.