Andrew,
I never heard about CL2.5. CLx (where x is 2 or 3) means that memory is refreshed every 2 or 3 CPU cycles.
If you look at web sites that sell Memory for MAC (
Edward you can find good deal here
Ramseeker or
DealRAM), you will see that they are CL2. Sometime you may fine CAS instead of CL but, the number and the meaning will be the same.
If you have the memory stick in your have, on one side of them, you should see a sticker which may reports the spec of the memory.
Hope this help
Mauro