all good points except one... have you ever riden in a hardtop supra? compared to a targa supra WITH the targa ON there is so much of a difference still. this bar ON with the targa on helps increase the rigidity etc....
i think your bar is great...however as you yourself have stated it is more for the cosmetic side with a little bit of function thrown in.... the one im producing will be 100% function FIRST...and it will be cosmetically appealing after.... even though it will be rather appealing when done
as for the headroom.... if you can sit with the targa on in your car and put 3 fingers between your head and the roof....this bar will be fine for you....
the cutting required to make it work is minimal and wont even be noticed if done properly and the bar can be coated or padded in many ways.
putting a cage in as stated above also would help but it STILL wouldnt not help the targa body flex because it doesnt mount in the proper points to do so.
hope that info helps.