Sure, why not? Unless someone argues the extreme pedantry of the name being "extended clip", and a drum not being a clip.
The core of the mod does this:
A Rating 1 Extended Clip adds fifty percent more rounds to the gun’s capacity, and a Rating 2 Extended Clip adds one hundred percent more rounds.
Note the usage of "gun's capacity" as opposed to "clip's capacity". I'm sure some will argue this on grammar grounds, but I'd certainly allow it.