This morning I ended up making a new makeshift rgc tester with steel bars, and it turns out that the gripper ramps up a LOT in the last 5mm. The final rating was 348lbs. Not sure how 400+ failed to close it. Here's some photos of it if you're interested. It's worth noting I had to tilt the gripper a lot to keep the strap from sliding off, but I think I did a pretty reasonable job of finding the sweet-spot angle.
Even more importantly, not only are the numbers comparable but they will be proportional, meaning you don't actually gain information by doing this. If you already have an RGC rating you can use that to derive what the rating would be by using a large or smaller strap, nothing new is being added.
It would be more helpful to quantify the spring constants to see how hard the sweep will be, as that and the spread are the biggest variables that make two grippers of the same RGC have different actual difficulties.