I have this option also enabled since announced and it works well to mee too (inbox with 51k items and 4.2GB)
initial loading of mails is a bit slower when enabled the converation view but thas ok (only about 1-3 sec slower as compared with this option disabled)
I just checked my multitenant server, and it appears to already be functioning this way in the WebApp. The Mailbox shows up as “My Name” (My domain). Maybe the setting is in one of the sections for Multi Tenant installations?