I took a perfect (artificially created) distortion chart.
Then I distorted it to 7% and corrected it back (manually - not 100% accurate but good enough).
I've attached a crop 700% enlarged that illustrates the effect.
The "perfect image" does not show any grey values - just black & white, of course.
You may see that one pixel row got pretty dark (loss of edge contrast). Or in other words - loss of resolution.
It's, of course, no surprise but it clearly shows how lossy a correction of a rather "typical" MFT wide angle distortion really is there.
How did you distort it to what your test software calls "7%"?
Photoshop uses a different metric.
You have to make a distorted chart from the outset, not make one, then distort it (because the distortion will lose resolution, if you do that).
This is what I end up with. I made a 6000 x 4000 image in PS. I put a chart in the top to "eyeball" about 6% using the Oly 12-100mm f4 PZ chart to judge the curvature.
I then pasted a pattern into the corner, of which you can see a part in the crops I made. I "undistorted" the chart again, and this is what the corner looks like after that.
Left is the pattern I pasted onto the distorted chart, right is the same corner of the 6000 x 4000 image after correcting for barrel distortion.
The shift you see in the pattern illustrates pretty well the actual resolution that gets lost.
Any tips on how to do this more accurately concerning what the test software calls "7%" are welcome.