Performance of DOMAINs

Howdy,

Didn't see anything in the archives, so I thought I'd ask: has anyone
done any work to gauge the performance penalty of using DOMAINs? I'm
thinking of something like Elein's email DOMAIN:

http://www.varlena.com/GeneralBits/

I figured that most simple domains that have a constraint check are
no faster or slower than tables with constraints that validate a
particular column. Is that the case?

But I'm also interested in how Elein made the email domain case-
insensitive, since I'd like to have/create a truly case-insensitive
text type (ITEXT anyone?). The functions for the operator class there
were mainly written in SQL, and if it adds a significant overhead,
I'm not sure it'd be a good idea to use that approach for a case-
insensitive text type, since I use it quite a lot in my apps, and
often do LIKE queries against text data. Thoughts?

Many TIA,

David