A targa will NEVER be as rigid as a hard top. Put the same cage in each and the targa is still, comparatively a noodle and is now even less desirable due to all the extra weight from the excessive cage... no matter what excuse you come up with for wanting to race a targa MKIII it's still a crap idea; Except for being able to wave at the spectators as you bring up the rear at every event.
I know logic will never stop anyone from doing it but stop spitting out logical fallacies to defend it.