Subframe Connector dilemma

Anything is better than nothing, the form fitting style would be all right for a street car.
Look at a 2X3 tubing this way..a carboard box is fairly strong when complete, some will hold an impressive amount of weight and have a strong structure BUT...cut the top flaps open and it's pretty easy to flatten or crush it.
I don't think those 3 sided ones pictured would be worth a whole lot with a set of slicks or serious horsepower. Even if the floorpan makes the 4th side of the box. 2X3 is the choice for a car with power, tied to the front and rear subframes and with the floor skins welded to them. Various people wil argue whether square or round tubing is better, I won't go there.
It's nice to be able to jack the car up with out the door gaps changing, and using a floor jack under them will lift the whole side of the car off the ground.
And that's something to remember if on a sloped surface, and suddenly you only have 2 points of contact to keep if from rolling!
As far as the ride quality goes, you might notice a little more stiffness, but the chassis not flexing might actually make it ride better in some situations. Still no worse than any A body ever was on a rough road.
Just my 2 cents worth.
(Added Later) Opposing opinions cheerfully considered.
Alan627b