You need to take into account that if you’re dealing with large capacity differences, more rpm isn’t always faster.
a 10TB 5400 rpm drive vs. a 1TB 10,000rpm drive for example - sure the rotational speed is faster, but the 10TB has 10x the data density and can read much more data per rotation. They also tend to have larger internal caches.
If you store the same 1 TB on both, the heads only have to move 10% as far in a random IO application as well, on the 10TB drive (and you’re also constrained to the fastest 10% of the disk! drives aren’t the same speed across the entire platter - the outer region is faster and they get slower as you run towards the inner tracks).
Sure that’s an extreme… but purely to illustrate the point.
X rpm vs Y rpm is only a valid comparison when you’re dealing with same/similar capacities. If you sacrifice capacity too much (just to get high rpm), you may well just be spending more/same money for less storage that isn’t actually any faster in the real world (and may even be slower!).
IMHO, super high rpm drives are dead. If access speed is important, you go either SSD cache or pure SSD to get far better performance than whatever rpm SAS. Because high rpm spinning disk is just a slightly faster turd - it’s still SLOW. Now SSD is cheap (even if only for cache) the use case simply doesn’t exist any more.
I say this as someone with both SAS and flash+sata arrays in my environment 
I wish we didn’t bother with the SAS.
