Funny, when I installed frame connectors on my Duster it was immediately noticeable. More positive handling response, less creaks and groans, doors that open and shut easily even when the car is up on stands. Sure, most of that is subjective, but an objective analysis of that stuff is not simple.
I would still install subframe connectors! Even assuming the post cars are stiffer is a bit of a mistake. The hardtops have a larger span without support, but that was taken into account with the design of the hardtop structure. Now, are the post cars probably stiffer? Probably, but it's still an assumption. And either way not so much that I wouldn't add subframe connectors.
Yeah it's not like a subframe connector by itself can remove all the flex from the car. It can't, the subframe connector itself will still flex. And just adding stiffening in basically one plane can only improve the rigidity of the entire structure so much
As for welding to the floor pan, gonna disagree. You can't know that. I don't know that for certain. I run tubular subframe connectors on my Duster that are just welded at the ends. I used large landing plates at both ends to maximize the amount of frame rail and crossmember I grabbed to ensure that the subframe connectors act over a large area, but they're not welded end to end. And there's an argument to be made there. MIG weld is more brittle than the surrounding metal. Only so much flex can be eliminated. It would be a legitimate question to ask if a fully seam welded subframe connector like the US Cartool connector doesn't overstress the area of the floor pan they're welded to. I think that long of a seam weld does introduce the possibility of cracks in the pan along its length. I installed connectors like that on my Dart though, so obviously I'm not too concerned about it. But it's a valid question.
I'm totally happy with the 1.5"x3" tubular subframe connectors on my Duster. I'm also happy with the US Cartool subframe connectors I installed on my Dart. And without a full FEA, I don't think you or I can say which one is actually better. Both are better than nothing IMO.
