I started many threads on this, you can search like docjohn suggested, most are about rollbars and are in the track section.
What it comes down to is everything is a risk, there is no clear answer but some setups are definitely safer than others.
A standard 3 point belt is actually quite poor. You can watch crash videos and the human body is all over the place. Arms and legs flailing, belt stretches a lot and torso flies very far forward. You are going to get tossed around the cabin like a rag doll. If your arms leave the cockpit (which they certainly will with the targa off), you can be what the pros affectionately call "de-gloved". Your hand will be severed, and john@microsoft knew someone personally this happened to at a track event.
A harness holds you in a lot better. Some will argue, but my personal opinion is that even a 4 point is much safer than a 3. The new Audi R8 GT V10 comes with 4 points from the factory. A 5 or 6 of course is the safest, properly installed in any kind of accident except a rollover with an impact severe enough to cave in the roof.
The B-Pillar on the NSX (and especially in the T) is extremely strong. Chances of that caving in is very small, probably 98% of accidents will not be like that. So in that high percentage, you are IMO, better off with a harness. In that small percentage where you roll and the roof caves in you may suffer a Severe/fatal injury. An NSX is very low, its center of gravity is low, and if you lower it, chances of a rollover are small. Then chances of a rollover where the roof caves in are even smaller than that.
I feel you are better off with a harness than without one. Properly installed. You don't want to be a projectile in the cabin. In side impacts the front airbag won't do much for you at all. A harness will.
To have that extra level of safety, the car needs a proper rollbar. If you get into a cage, you will have issues on the street in accidents as your skull can crack on the bars. A rollbar, being behind you is OK. That will add extra strength to the B-Pillar and even a rollover becomes less of an issue.
I've only seen one proper rollbar for the NSX. The Comptech. A proper rollbar needs points of attachment fore or aft, not just be a "hoop". The aft points would be the rear strut towers, for that the worst thing is the rear glass has to be replaced with lexan. Now you have really altered the car. The front points are down low near the front edge of the doors where the speakers are. This is the route Comptech took. This is still not approved by many racing bodies, but we are talking basic safety not competition regulations.
STMPO actually designed a rollbar for me but it was never finished, Ross and I will probably revisit the issue next spring. This is a summary but there is a lot more detail in my threads.
