(Source, I'm the creator of Owl Mail [https://owlmail.io] and this is a common question.)
By using Owl Mail (or Firefox Relay, etc.) addresses everywhere, you reduce your attack surface to one security fastidious company.
And, even if Owl Mail (or Firefox Relay, etc.) were to experience a data breach, at least it would greatly increase the effort required to match emails to your identity.
To really protect yourself, use a double relay!
External -> Relay 1 -> Relay 2 -> Your Inbox
Also, I think one thing you should look into as a natural evolution is promoting the use of auto-generated, secure passwords unique to each relay address.