You can use an routines in untrusted PLPerl or untrusted PLPython, but these routines can be really unsafe (due possibility to break signal handling).
Personally, I think using http access in stored procedures is a very bad idea - access from transactional to non-transactional (and possibly pretty slow) environments creates a lot of ugly problems. Stored procedures are great technology with a pretty bad reputation, and one reason why is usage of this technology for bad cases.
I think this mailing list is wrong for this kind of question. There is no relation to performance.