Upgrading to the VIIC in real life would give you more reliable electrics, greater range, a slightly faster dive time and probably a happier crew too, since it had more room inside. Additionally, later variants of the VIIC with the 'Atlantic bow' were more seaworthy and had a greater diving depth capability as well as greater resistance to damage, owing to the thicker gauge steel used in their construction. Then there is also the 'sixth tube' to consider; confering the ability to use pilenwerfers and decoy torpedoes on later boats, was a real advance. Whether any of these factors are in the sim - or any mods you might have - to any great degree is debatable however.
I can understand you being attached to your boat though. But for me, being able to go deeper - and do so quicker - outweighs any advantage that a tighter turn confers, since greater depth extends the transit time for the depth charges, time in which you can be 'hauling ass'. Like in fighter planes, turning ability is more of a defensive attribute than an offensive one, a nimble fighter might be able to out-turn an enemy, but a faster one can break off and re-engage at will. Of course this analogy is not fully comparable with U-Boats when we are talking about destroyers that can outpace us all, but short of turning around to fire a single stern shot after having expended all your bow tubes, to me there seems little offensive reason to value a faster turn.
But, if you like your boat, and it suits your style, maybe that's a good enough reason above all others.
Chock