That is how I have always understood it to be.
With stock seats the regular seat belts are safest. In event of a roll over the occupant and seat do allow enough movement to not have the roof hammer against a fixed locked head and neck which would be the case if one had harnesses.
With a fixed shell seat, you have to have a 5 or better still a 6 point saftey harness. But you would need at least a roll hoop or cage to keep the roof away in case of a roll over. But then you would need to remove airbags (because they are designed with regular seats and seat belts in mind. Plus many race seats are at different heights than stock seats).
The schroth harnesses are only aids for driving (holding you firm in the seat during hard cornering) and dont really increase saftey.
So I am of the opinion that it's best to keep it stock and if you go harnesses or seats you have to go all the way and include hoops/cages/seats/harnesses as a system, as mentioned above.
