Subframe connectors stiffen the frame of the car (unibody). So yes, they do make the body stronger. And my only thought on the link you are looking at is that you might just want to look at Global West or Maximum Motorsports connectors before you weld those on. Since this is a weld on modification, you only want to do it once - so don't skimp unless you will be confortable with that for a long time.
As far as this discussion of subframe connectors reducing body roll, I'm a bit confused. I could maybe see where you are coming from given that SF connectors reduce flex and allow the suspension to do its job better. But the biggest difference you will perceive with SF connectors has nothing to do with roll. The car will feel 100% stiffer over bumps and irregularities (especially if you have a 'vert). You will notice this in how the car squirrels around less, and how the front and back of the car no longer feel like they are moving independently of each other.
The modifications that WILL reduce roll are lowering (or coil over), poly bushings, and improved anti roll or sway bar upgrades. High quality shocks/struts can have a big impact on how the suspension responds - possibly reducing roll as well. In comparison to these mods, SF connectors do NOTHING for reducing roll.
Hope this helps.