I have investigated and tested this issue extensively across my fleet of skis and boots. I have a lot of skis, let's just leave it at that.
First, the major issue that must be understood before you assess any ski binding's lateral rigidity, is that the height of the rear heel lug of your ski boots is absolutely crucial to having a strong interface between the boot and binding. The ISO 5355 standard states that the heel should be 30mm +/- 1mm. However, in my testing I have found that ANYTHING less that 30mm will result in reduced lateral rigidity for any binding. I use a digital caliper (with plunge depth) with a triangle square for accuracy when measuring the heel height. If the heel is below 29mm - just forget it... you really need to either plate it or shim the lug.
The toe lug height is less of a concern for the rigidity issue because it must have some clearance for the binding release to work properly. The ISO spec is 19mm +/- 1mm, but almost all techs will router a toe to 20mm (the maximum allowed). 'Just know that lateral rigidity is "born" in the heel.
That all said, there is an absolute difference between bindings types when it comes to lateral rigidity (as the OP noted). The most rigid setups in my fleet are those on race plates (or Tyrolia/Head Speed plates) coupled with a high DIN race-type binding (think FreeFlex 14/15 or higher OR the new EVO series). The PowerRail series bindings aren't bad, but they are not as laterally stiff as a race plate setup. There is some movement in the rail system. The PR is a bit better than the older Railfex II system. Note that the width of the plate/rail also has some impact on the "stability" of the connection. Wider is better.
BTW - The way I test this is by clamping the ski down to my tuning bench (don't use the ski vises). I use 4 large Irwin clamps with 2 directly next to the binding's toe and heel and the other 2 at the ski's tip and tail, so that the ski is flat on the bench. I then put the boot in the binding and measure the angle from between the ski surface and the rear spine of the boot (lower clog). I then push on the boot cuff laterally and note that amount of movement seen. I have detailed data on this stuff.
At the end of the day, whether or not you notice this (or it's important to you) depends on your goals and the ski conditions you generally ski. Obviously in softer conditions this probably is not noticeable and less of a concern. The harder the snow and the wider the ski, the more this becomes an issue.