Indeed, one of the main advantages of having the primary key indicate the layout of your data on disk is that you can optimize access to your data for the least

Indeed, one of the main advantages of having the primary key indicate the layout of your data on disk is that you can optimize access to your data for the least amount of filesystem page cache misses by making the primary key be the column (or combinations of columns) that you know you're going to have predictable batch access to. Benchmarking that can be tricky for the same reason that benchmarking anything that reduces cache expiration can be tricky. If you reduce cache expiration your query might not benifit, but if you take a holistic approach to it and apply it to the whole database it'll benefit as a whole. Any big MySQL powered application is likely to make wide use of this. Which goes to show that doing any benchmark comparisons between databases is often meaningless. You can't benchmark the same schema/queries between databases because they'll be optimized for the database you started out with.