If you're playing with the GWX mod, I'd say you're better off saving your renown points and skipping over the VIIC completely to the VIIC/41 (with the optional 'GWX VIIC/41 playable sub' mod enabled to get all the proper stats). The VIIB is adequate for the job until 1942, anyways, but improvements in enemy ASW technology and tactics make survival a tough proposition if you don't upgrade and the VIIB is very limited in what can be retro-fitted.
The VIIC/41 has a thicker pressure hull and can dive much deeper than either the VIIB or VIIC. I think the crush depth is around 280 meters, versus the 240 of the VIIB and 260 of the VIIC, which means you can easily dive to between 240-260 meters before you risk damaging your boat. The VIIC/42 can go even deeper still.
Going as deep as possible can save your skin when those escorts are hammering away with depth charges and locked on to your exact position with ASDIC. (If only thermoclines were modeled into the game!)
Despite the GWX subs having the historically listed deeper dive stats versus the much shallower depths in stock SH3, it's somewhat annoying to have your CE babbling away about the depth long before you actually do reach critical but that's apparently hard-coded in the game.
I guess you could look at it as your CE and Hydro/Sonar man are just being nervous old ladies.
I've never been a fan of the "Seekuh" type IX series boats, preferring the more versatile size and maneuverability of the VII series. They were nicknamed "seacows" for a reason. They do tend to steer and wallow around in the water like their namesake but those big boats do have their charms such as long-range endurance and enough eels to satisfy all but the most bloodthirsty kaleuns.
I've read that a few were even spacious enough to include freezers for keeping fresh meat and other perishables on hand for those extra-long patrols of which only the IX was capable.