Hi cy,There are no after market upgrade products available for the M30. If you want to replace the struts, any Infinity dealer can do the work. I had mine replaced a couple years back by Infinity Denver for $600 for the two rears. I found a site dedicated to the M30 at
http://www.phpbbforfree.com/in...rapps if this link does not work do a search for M30leopard or email me. A couple guys at this site have installed 280z springs and struts to replace the OEM struts and throw out the sonar suspension gear. They may be able to help with upgrades if you dont want OEM struts.
Good luck with your M30, Steve