I love the ability of IXD2 to shoot down enemy airplanes. In last mission, I shot down 91

of them, sank 93k tonnage and brought back 5137 score. I wouldn't trade my IXD2 for VIIC back. I used VIIB before VIIC and liked it. VIIC didn't really provide me any advantage, except for faster diving time. VIIC can reportely dive deeper. I got rid of VIIC in february 1943 when IXD2 became available. For VIIC there are nice conning tower upgrades, so you can have the same flak defence like IXD2 - 1x quad and 2x double AA. But I think this becomes available later, and I got tired of VIIC.
Btw I don't know if you found it out, but at least in stock SH3, you don't have to go to your patrol square, just go anywhere you want, sink everything until you run out of torpedoes, then radio back results, and they will tell you to return to base. This saves you weeks in a IX boat.