pr00ne,
There's more to an aircraft's gun than just its calibre. Your comparing the A-10's to "other 30 and 25/27mm cannon equipped aircraft" shows you don't understand that.
Excellent point. I'd like to add to that. There's more to CAS than just strafing. Strafing is just ONE way to do CAS. There are many more. And with the advent of precision guided munitions (both unpowered bombs and powered missiles) strafing is often a secondary method of providing CAS. So while the A-10 is the premier strafing platform with the capability to use many other CAS weapons, many other aircraft are excellent platforms for those other CAS weapons.