If the replication factor for blobs is calculated from the 'hotness' of the data (by looking at the age for example), I'm wondering how CDNs come into play.
Increasing the replication factor would allow for faster reads, but there usually wouldn't be a need for that if you offload reads of hot data to a CDN service (could be an internal CDN...).
There is a just-before-storage cache (through which the CDN edge requests) which reduces requests per object, but it does not affect the number of hot objects per disk. With Haystack each object is fully available with sane performance in degraded state. This means 1/3 I/O per disk, whereas F4 only has 1 easily available full copy, which means 100% load on one disk per object stored.
Increasing the replication factor would allow for faster reads, but there usually wouldn't be a need for that if you offload reads of hot data to a CDN service (could be an internal CDN...).