HiI also agree, that speed is always strongly demanded commodity.
However, learning from XML database implementations, I would be careful with taking enough space as granted.
My original preferred XML database implementation was Sedna XML database, but space requirements were sometime too high like 100 times the space of original docs. They were giving some hints, how to make it smaller by restructuring used XML structures, but this completely destroys the beauty of XML databases offering solution for just existing documents one has at the moment without any additional constrains.