It will help people and me to find out if the kit is worth the money and installation time. For me this kit makes only sense if it will really centers the subframe, so that a good alignment will be achieved, but not many talking about it.
It's going to vary from car to car. By how much? Probably not a ton. If you're able to get it within 0.1 deg camber left vs right you're doing pretty damn good.
I have a ton of measurements that I took a while back when attempting to model the suspension (which I've been procrastinating on as it's low on my priority list right now)... but based on some quick back-of-the-napkin math, shifting the subframe should net approximately a 0.1 deg change on BOTH sides (lower on one, higher on the other) for every mm you shift it left vs right. I'm guessing there's *maybe* 2mm of play total on most cars when you take into account that not all 4 bolt holes are going to 100% perfectly line up with the holes in the chassis, which will eat into some usable wiggle-room.
I think the subframe collars are snake oil for what they're advertised as (better handling because they're perfectly centering your subframe and making it more "rigid" or whatever), but I don't doubt they can make removing and reinstalling more consistent if you're doing that often, but alignment (at least toe) should still be checked/verified when done.
If you're paying someone else to do the work - then I'd skip the collars and just make sure they take the time to verify everything is square and do the final 180 deg portion of the torque sequence on an alignment rack.
Also most people don't understand that on an alignment rack - you can pull the car right off, and immediately right back on and get a slight variance in readings. Hell, the way the alignment tech pulls the car on will have an effect as well. It's shockingly common for some techs to not pull the rear locking pins on the slip plates causing the rear to bind up. Plus the way the tech installs the alignment heads to the wheels followed by performing the rolling compensation will affect things as well. Even a $100k alignment machine won't make up for the lazy or complacent operator. Also alignment machines being out of whack and having some tilt in them is also hugely common. all it takes is leaving the chassis jacks in the down position once while lower the entire rack to ruin the levelness of the entire thing. Or idiots to hold the alignment heads wrong. Or drop them off the car 5ft to the ground because they didn't use the safety loops through the wheel spokes after clamping down on the rim edge.
I can't say I have direct experience with the locking collars (though a trusted friend and VW tech tried them on his MK5, and he could not get the alignment right so removed them)... I'm just saying that people who claim their subframe are shifting are probably either at fault themselves (reusing hardware, improperly torqued, or reinstalling the subframe with the chassis "twisted" on jack stands causing the subframe to be in a slight "bind" after it's been fully torqued and then dropped onto the ground.... OR they're going to different alignment places, on different racks, or two technicians who have different or inconsistent procedures, or didn't set the tire pressures prior, etc. etc. and the list goes on.