The 425 belt is a bit wider, and maybe something to do with the molding process may be different. I think they starting in model year 2012, they started using the 425. Previous to that, the 288 was used. A lot of teams continued to use the 288 though after the change Since they are both so close dimensionally. Probably more due to keeping things familiar. For my 15, I run the 288 belt based on the previous owners suggestion even though the 425 is what is called out for it. It has worked great.
Also, seeing the 2016 RS has gone back to the 288, I figured what the heck. I'll keep running the 288.
Both belts as far as I know are great belts. They are also close enough dimensionally to run either one.
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