Otto, fighting C3's at higher war level is (or should be) more profitable if you are trying to profit from C3 wars. Also, I'll be fighting C3's at a higher war level just for the higher level challenge (currently I'm staying WL3 until my empire stabilizes a little.) I did what you want to do - expanded to 10 countries by fighting C3's. It worked great. I now fight them for the cash and experience.
The above is my experience - below is my assumption...
Some other reasons for higher war levels:
Some gold coins each level
More challenge and opportunity to fight other players at higher levels (if that's what you want - also means higher level players can fight you!)