For comparison, Bugzilla sent all bugmail from the same email address but did not add a sender name.
Personally I appreciate Phab's behavior because I have one item less (the actor) to filter on as it's already displayed in my mail client.

this confuses mail clients into thinking all mails are from the same person

Which mail clients except for the Gmail web interface are affected?
I rather consider the behavior that you describe a potential bug in Gmail, to be honest...

Ideally, email addresses in the From: field would look like

[username] <no-reply-[userid]@phabricator.wikimedia.org>

This would break filtering incoming mail by the criterion "sender address" and isn't desirable.
Using a plus sign as a sub-addressing tag might work in some MTAs and MUAs though (RFC 5233, no idea about Gmail).

In T77681#832223, @Aklapper wrote:

For comparison, Bugzilla sent all bugmail from the same email address but did not add a sender name.
Personally I appreciate Phab's behavior because I have one item less (the actor) to filter on as it's already displayed in my mail client.

It's certainly an improvement to bugzilla, but in UX issues that's a fairly low bar :)

Which mail clients except for the Gmail web interface are affected?
I rather consider the behavior that you describe a potential bug in Gmail, to be honest...

IMO it's reasonable behavior in to conserve space in an extremely dense view, given that people who use different mail clients e.g. at home and at work often use different names for the same address. At any rate, given that Wikimedia uses Google Apps as an email provider, and Gmail is probably the most popular email client in general, it's reasonable to expect workarounds for bugs in Gmail. (If Phabricator did not work properly in IE 9 because of something that browser did not get right, would you say that should not be fixed because it's a bug in the browser and not in the web application? We are speaking about comparable market shares.)

Ideally, email addresses in the From: field would look like

[username] <no-reply-[userid]@phabricator.wikimedia.org>

This would break filtering incoming mail by the criterion "sender address" and isn't desirable.

Is there any situation in which simply filtering for @phabricator.wikimedia.org is not suitable or not supported by the filter software?

Using a plus sign as a sub-addressing tag might work in some MTAs and MUAs though (RFC 5233, no idea about Gmail).

You mean like this?

[username] <no-reply+[userid]@phabricator.wikimedia.org>

What do you mean by "works"? That different usernames are not collapsed in conversation view? That filtering for no-reply@phabricator.wikimedia.org will match no-reply+foo@phabricator.wikimedia.org?

In T77681#944054, @Tgr wrote:

Is there any situation in which simply filtering for @phabricator.wikimedia.org is not suitable or not supported by the filter software?

True - I'm not aware of any other local parts currently used (locally we've only used PhabAnnounce@phabricator.w.o so far but that was a bit hackish).

Using a plus sign as a sub-addressing tag might work in some MTAs and MUAs though (RFC 5233, no idea about Gmail).

You mean like this?

[username] <no-reply+[userid]@phabricator.wikimedia.org>

What do you mean by "works"? That different usernames are not collapsed in conversation view?

I don't know. I neither use the Gmail web interface nor some mail client that allows a "conversation view" and I don't know their implementations.

That filtering for no-reply@phabricator.wikimedia.org will match no-reply+foo@phabricator.wikimedia.org?

It should not. But again depends on the filter implementation of your application.

In any case I personally don't plan to investigate providing requested functionality locally. Plus I'm not convinced that this is a good approach at all so I'd decline this.
If you feel strongly about this feel free to talk to upstream (on your own risk) to get some more input.