Long time lurker, first time poster. Inspired by the excellent work of Storker I tried the same trick with my setup - EM-5 in the Oly housing with Zen dome and adapter. Shots run through LR, tone adjusted with auto (sun kept moving behind clouds) and a quick distortion correction for the +2 and +4 diopters. I also shot freehand with the chart stuck to a pool wall so the same caveats around accuracy apply, however I think you'll agree the differences aren't subtle at the 9 mm end (corner crops):
The +4 diopter didn't gain much on stopping down, the +2 caught up with +4/ f5.6 at around f10. The CA cleans up in LR nicely even on the +4, the diopters were cheap close up lenses I brought about 10 years ago so it may be nicer with more expensive glass (or maybe not). The corners were much nicer at the 18 mm end for all settings.