I'd agree with Matt, many of the variants appear to accommodate specific Amiga models. For example, later 8375 versions added a 14MHz clock output that was needed for Gayle that was used in the A600. It also appears that they might have been trying to work around limitations of not quite having enough pins on the PLCC84 package to do everything.
Some variants of 8375 are the same die internally, but bonded differently to change functions of specific pins. Have a look at this specification document:
http://amiga.serveftp.net/Datasheets/390544-01_8375_Agnus_specification.pdfNeedless to say, many of the variants have different pinouts, so they're not always directly exchangeable.
I'm not aware of anything major that the 8375 'fixes' from the 8372. Though I am aware of the many headaches I've had with repairing boards from customers who have helpfully decided to play "Agnus Roulette" by swapping out some random variant of the part, can't get the board working, and send it for repair, neglecting to mention that wee gem of information. I then spend hours trying to figure out what's going on with it, eventually discovering the aforementioned game of roulette. In addition to whatever the original fault was.
My advice is that if the current part is working, then don't mess with it.
