The extendibility has not been tested, although this seems to be one of the feature of the library, which issue is dealt with in part of section 14 of the User Manual (See Bibliography).
Performance not evaluated
It does not seem to be supported by the developers. The installation of COOL was a tragedy: several problems were found, ranging from files not found to compile errors to link errors, etc.
After a couple of days of trials the installation was abandoned, although a post was made to the net to see if somebody could help on that.
I got some answers after some time and I found out that other people had similar installation problems. They suggested some solutions which I haven't tried yet. It is interesting to note that the authors of COOL have written a paper on which they analyse the lessons learned from the usage of the library, what they would keep of it and what they would change in its design, and so on (See Bibliography).