What is the SQL Server cost, in terms of performance, of sharing physical disks?
A performance factor which we saw in one of our environment was related to  high disk read write activity from one database on those shared disks. Since one had a higher transaction activity using the storage hitting hard caused slownesss for other. Also it becomes hard to troubleshoot when you have shared instance for those disks. We moved them to their respective virtual servers instead.

Also other factors which made us move was difficulty in performing any reboot activity after patching as one lob would be fine while other conflicting.

Moreover backup of databases in one instance at x timings may impact performance of other sql instance depending upon how big the servers we are talking and how good or bad the storage is.