I am trying to come up with a test scenario regarding these updates. So far the best thing I've seen is in some video TOC emailed me links to. A live situation may not be definitive. Fish move, the boat moves and in my case I may have moved the TM slightly. What I propose to do is roll back the software with a forced update to v16.00/v2.10. Then perform the test as objectively as possible through each successive software update. So that would be:
v16.00/v2.10
v17.10/v2.20
v18.10/v2.33
v18.10/reversion to v2.20 gain tables
For each test, care would need to be exercised to use the same settings, same location, same targets, etc. My units are GPSMAP units so a video of each test could be captured via ActiveCaptain. That works most of the time, but I have had AC videos that turned out pixelated and garbled that was different than what I saw on the unit. That's just a chance I'd have to take.
So far the things I see reported to look for to document are:
Ghost Trees
Reception differences
Dead spots
I'm taking suggestions for any aspect of this.