I would think that they would still help, but I bet the hard top does help a good bit. I have a roll bar now as well, but it doesn't help much with marrying the front and rear of the car together of course.
True, imagine how much more traffic could safely cross if they added more lanes too! I could stand there to be a few mcdonalds on the way too, I was getting hungry.
I think, the differences between measurements might have been dude to your measurement mythology. I think a simpler way would have been to not use tape and pins, tie or bolt a string to each point you wanted to measure, pulled them tight with some sort of luggage scale, so you would be sure the force exerted is the same. Then jack the car up or what not and see if the string sags. Mark the original point that was bolted down pull the string once again with luggage scale to the same force where its straight again mark it and use calipers to measure the difference. I honestly from an engineering perspective cant see these really adding any stiffness if any thats noticeable. The material used to make this also matters. If this did create any sort of rigid connection after a few years you would see a massive dent wherever the 3d printed piece came into contact with the sheet metal. The mazda one i am pretty sure is to remove any sort of vibration or sound from the door being bolted on, not to increase vehicle rigidity. cool thoughts though!
I thought about strings but I think the way I went offered more precision. I made it pretty clear (with data and driving impressions) that it does in fact make a noticeable difference. The seller of this part originally tried ABS plastic but the bushings would crack, so there is certainly a significant force applied to them. Also, when the Miata windows are down and the door is closed, it sounds pretty cheap and flimsy. I don’t think the Mazda bushings are for that reason.
Lifting both rears is exactly the same as parking it on a hill, doesn't impart any bending force. The twist test is really interesting though. Another way to measure this would be to put the car on corner weight scales and then put blocks of wood under one wheel and compare measurements.
