Can't help w/the 54" mower but I can remark on adapting a set of gatorblades for use on my
BX23's 60" MMM. I'm one that had to use a Cat 1 to Cat 2 bushing, cut down to approx 1/4" thick (just slightly thinner than the blade) to adapt a set of std GB's to my MMM and after approx 400hrs and now going on the 5th season, mowing once a week from mid March to late Nov, I've had absolutely NO Issues with this setup, none....
I am so impressed w/the performance of the GB's that I upgraded this season to the 3 in 1 Hi lift Fusions in hopes of getting more lift and hopefully an even batter cut. The juries still out as the lawn hasn't completely filled in so it's hard to tell but it does seem that they cut through the thicker growth easier than the std GB's did.
I'm also impressed in how the std GB's hold their edge.
re. OEM mulching blades. Theres been many that have posted, both here and on other tractor sites, that the OEM mulching blades produce as good if not better results as the std GB's do. Those that have posted these results did so w/just the OEM mulching blades mounted and not the mulching kit. If going w/a bushing and/or the GB's still not having the correct spindle hole size is still an issue, ordering just the OEM mulching blades MAY be a viable solution to improve your finish cut. They have higher lift and additonal cutting edges over the std Kubota blades.
re. bagging and side diischarge mowing w/mulching type blades. I side discharge as well as bag (w/a cyclone rake) and the performance of the GB's has been nothing less than great, much better than the std OEM blades that came std on my 60" MMM.
Since these blades cut the clippings, leaves etc up finer, I can cover more ground inbetween dumping the bagger and they compost easier, win/win...
There are also some that have posted that they didn't see any improvement when they went w/the GB's and wasn't impressed w/them. Like every thing out there, theres differing opinions on everything, so I 'm throwing mine out there too
Good luck in the quest for this elusive 54" GB, I believe y'all are on the right path

Dave