I find that Calibre gets very sluggish moving between rows if your cover images are too big (certainly with the book details panel visible). It is another reason for resizing them down.
I disagree about "needing to find another image" with the Overdrive size of 510x680. I find that those covers just need resizing in their aspect ratio, so that they no longer look stretched (certainly for the covers I have seen it appears they were just stretched into that 3:4 ratio, not that they were especially cropped into that size).
Personally I don't actually care very much about the image inside the book, which is not what this plugin is trying to address in any way. As a Kindle user you don't get to see it by default anyway, and even if I did see it who gives a crap about something you are just going to click past. OTOH I do spend many hours browsing my books in Calibre, and I know that one day we will have a "proper" cover browsing option which will look a dogs breakfast to the OCD types like me who get upset about things being all different sized shapes on the screen.