Definition editing gadget
The tool was tested quite a lot, although most of that was a long enough time ago that it's possible that new problems may have popped up as a result of the many changes to Mediawiki since then.
There was a vote on enabling the tool for all users a while back (Wiktionary:Votes/2012-10/Enabling Definition Editing Options), which failed to pass. Enabling it by default for all users or specifically all registered users is indeed possible, but I think that excluding IPs misses most of the point.
Conducting a test sounds like an interesting idea, but it might be hard to gather enough data to determine whether/how much the tool encourages additional good contributions. Using the Tag filter/edit filter to tag edits done with the tool (so that the recent changes can be filtered to show specifically those changes and such), might be helpful. I'm not sure if that's possible/feasible, though.