Nice use of XML for data manipulation. I have not tried anything like that yet. My next task is to try an XML-type column to hold a variable number of 'keywords' related to the row, plus experiment with an XML index for retrieval.

In terms of trade-offs between maintainability and performance, that can go either way depending on circumstances. In the systems I have developed over the years (mostly as an ISV), making maintenane easier always won.

The original industry-wide airline reservation system was very difficult to maintain, because they had to optimize the heck out of it to service tens of thousands of terminals and hundreds of TPS on the available computer power of the day. That's why airline codes are 2 and exactly 2 uppercase only alpha characters, airports are only 3 chars, etc. as they squeezed out every bit possible. It used a custom file system missing most of the niceties we come to expect from a robust RDBMS. Not a system I would like to maintain.