First, you need to keep in mind that the aggregate is the performance boundary, not the Volume. In your picture above, all DB's are always in Aggr1, so there will be no performance difference. In general, NetApp best practice is to make your Aggregates as large as possible as this will allow WAFL more spindles to manage and give you the optimal performance across all your LUNs.
The main reason to separate your DB's onto separate Volumes is to separate your snapshots. Snapshots are taken at the Volume level, not the LUN level. If, for some reason DB1 was on a different backup or retention schedule from DB2, it might make sense to separate them onto different volumes.
we'd quite a lot databases in a configuration of a customer hitting the same question. We did the test with around 20 Databases on one node and two different volume layouts.
first layout was 20 LUNs with 20 Volumes
second layout was 20 LUNs with 1 big Volume
The basic disk performance was absolutly the same as stated before as the disk performance depends on the aggregate not the volume layout.
the backup performance was a bit different as the process of doing 20 snapshots in a row takes much more time than taking just one snapshot. Even if SME is not aware of this configuration and verification always flexclones for each DB (deletes if verify for the first db is ready and recreates flexclone again for the second db.. which is also quite a bit overhead and time consuming)
Backupjob as a whole completed (without verification as it depends on the db sizes) with first layout after around 15 Minutes, second layout took around 9-10 minutes. not that much and especially if you do verification the 5 minutes more or less will not count in the whole process. (tested configuration with SME 6.0 no P release or 6.0.2 as the config was a bit older and never updated, SDR 6.3 was in use, also no P release)