Hi Mansi,
No one answered so far, so there is probably no simple answer to your question.
As you indicated, many relational DBMS (such as DB2 and PostgreSQL) provide support for XPath as well, but in most cases, the systems do not create additional index structures on top of the XML nodes to speed up querying. However, as the queries you presented on the mailing list are simple XPath expressions, there are probably not so many chances to benefit from values index structures anyway.
If teams in your companies are trying DB2 and PostgreSQL anyway, I'll be looking forward to hearing more about their experiences. Commercial systems often disallow their users to publish performance results (and presenting them on the list is some way of publishing them), so you can also send them to me privately. If you go for PostgreSQL, this won't be a problem of course.
Hope this helps, Christian