You've definitely got a good enlcosure so I know for sure that's not any sort of issue.

Adding the 8's won't make the difference you're looking for as it seems what you're looking to do is get more midbass. Correct? 8" subwoofers aren't the way to go to achieve that though. Adding more drivers to help get more output in a specific range isn't the correct way to achieve that goal.

You've definitely got some nice components in those C3's, but how are they installed? If they're not properly installed you are most likely not getting everything from them they have to offer. Custom baffles, proper sound deadening and tuning are required no matter how good or bad a set of speakers are, the difference is in how much tuning they require.

You mentioned two sets of the C3's, do you have them both in the front or one set in the front and the other in the back? If they're front and back you should try moving the fader to the front only and see if that helps the midbass response. You should also try reversing the polarity on the rear set to see if that helps strengthen midbass response. If they're all in the front try unhooking one set at a time and seeing if you notice a difference in output, there's always a possibility of having one out of phase or cancellation between the front rear sets if that's how they're installed. Sometimes it takes a little extra playing around to get the best response from anything, no matter how good, bad, expensive, inexpensive, large, small or well reviewed.

Adding a set of 8" subs to your 10's won't gain you anything and in terms of how they'll sound, well, you'll likely never know that they're running because they won't keep up with the 10's in output. Think of it like the cafeteria at school when everyone's talking really loudly. If a single person talks in a normal voice they're almost impossible to hear even when you're close to them. There will ALWAYS be someone out there who's done this and will SWEAR they heard a difference and that it "made a huge difference". It's just psycho-acoustics at play. They WANTED to hear a difference, so they did. The same way that the guy with six 6x9's laying in his back window, four speakers laying in his dash, an assortment of different subs all in different boxes with wires and crap everywhere thinks his setup is the beat all end all. No one who spends money on this hobby wants to be disappointed with results.