Storage Pools and Number of Disks

Our company has a CX4-120.

Documentation states best practice is to have a maximum of 20 disks in a storage pool.

In our case, the storage pools are configured with  5 x 600GB 15K and 15 x 1TB 7.2K drives.

If I were to setup my storage pools with a 10 x 600GB 15K and 20 x 1TB 7.2K drive configuration, what is the downside?