The filing down should work fine. I got mine when they first came out and haven't had any problems with them fitting my G3 variants. I haven't noticed any effect from the lack of material. I can't say for certain about the CA series as I've only used their metal body kits and not a CA built AEG; they may fit without any modification?
I found something on the WAA boards from back then that I posted when working on it. The original post is missing the pics another guy took so it may sound vague. Here's the original link:http://www.wiairsoft.com/forums/showthread.php?5584-G3-Midcaps
"I just filed mine down, but haven't loaded them up or used them in a game yet (I did fit them in both my MC51 & G3 metal bodies). I ended up taking off about 1.1mm from the top front and back (total). The upside down "U" in the front top can be sanded down. Also, on the back, top, is that piece of plastic that Doc saw; that was sanded down also. If you look at Doc's pics the one where he has the KA & TM side-side facing you, the top horizontal plastic is what I sanded. When you fit them you'll see where it's rubbing, it should leave a glossy mark if you give it a swipe with sandpaper. From my modeling stuff I used the "coarse" flexipad and just sanded, fit, sanded, etc until it got to where it's tolerable. There's still a little push needed but that shouldn't be a problem; more can probably be sanded off if it's an issue. The mags have a solid "click" when inserted all the way, stayed in, and were easier to remove than insert."