This is something I have wrestled with for some time. I would love to have the "hug" of a harness setup for tracking and the Schroth 4 point harnesses with anti-submarine technology I believe would be the way to go if not switching out to a dedicated race seat. Bonus is that the Schroth's are street-legal.
However, I have discussed this independently with a couple of pro drivers, Brian Lock and Billy Johnson. Both have discouraged me from going with a harness bar alone and would favor the install of a quality 4 point roll bar with incorporated harness mounts. They both grant that the majority of on-track accidents are unlikely to be rollovers and therefore a harness bar and harnesses would be safe in those events. But if one does have a rollover, which is certainly not unheard of, and the roof collapses, you will be much worse off if you are held vertically by the harnesses than if you were using the OEM 3 points, which would allow your upper body to be moved to one side or the other. Therefore, I have held off until I decide to make the plunge for a well-constructed weld-in 4 point roll bar. Until then, it is the OEM 3 points for me.
YMMV, but this is the route I have personally chosen.