Halli Hallo
First, welcome in this board!
The only you can get with changeing the "Computer Programm" (indicating mesurment field) is about 12BHP.
The japanese version is using other camshafts. They are grinded more acrimony plus the other "Computer Programm" (also called "engine handler" ) .
May you can register you in this forum and tell us more details about your Xedos!
Hatschi
Ps.: sorry about my bad english...