The lowering perches don't work very well for XI's. See the thread below that I went into more detail in. They have settled a little bit more after a month, but are still way off from advertized. I think one of the problems is the new tophats. If you remove a 3 year old tophat and replace it with a new one, the new one is going to sit higher. It will take time (probably a long time) to compress the rubber in the tophat to the same level as the old one. The non-xi people see the immediate drop since they are using their old tophats with the perches.
http://www.e90post.com/forums/showthread.php?t=780476