I just remembered I hadn't updated this with a taste report!
To me, the Gros Michel is slightly better in every way compared to a homegrown Cavendish. I compared the two side-by-side again this morning and the GM has more "banana" flavor with none of the extra slight grassy notes that are present in the Cav.
Additionally, there is just a higher concentration of flavor in the Gros Michel. I would say that a homegrown Cav is itself significantly more flavorful than one from a grocery store, but taking a bite of the Cavendish after eating the GM, it tastes like it has been diluted somehow. The GM also has a very subtle hit of acidity that really nicely balances out the sweetness.
Texture wise, the GM is juicier and creamier and sort of melts in your mouth as you eat it. This is, to me, a big area of improvement over both homegrown and (especially) store-bought Cavendishes, which can have that dry and pasty texture that isn't very pleasant.
I have noticed that as they get slightly overripe the distinctions between the GM and Cav shrink (Gabe mentioned this in a comment on my Instagram a week or so ago and he is absolutely correct). The GM actually does get somewhat pasty when it starts getting significant brown spots on it. At this stage I would say that the two aren't notably different.
Overall though it is really good. Definitely among the best bananas I have tried, and a clear winner for me in the "bananas that taste like a banana" category. There's nothing weird about it flavor-wise - it doesn't taste like apples, or berries, or pineapple, or (thankfully) banana candy - but it is just a really good banana, and I'm very glad to have been able to try it.