Ok, I have tried to contact the xcache devs, but apart from "xcache won't work with fastcgi", all attempts to get something to work have failed.
I did find a blogpost (after much searching, too) that tells about FastCGI with a PHP Opcode Cacher. Might this be something interesting for ISPConfig 3? The blogpost is at..
Basically it suggests to not use mod_fcgid but to use mod_fastcgi, together with pecl-apc. There are advantages and disadvantages, the blogpost lays them out.
Would implementing this be interesting for more ISPConfig-3 users? Please shine all your christmas lights onto this subject.