Well, I'd be concerned about inability to track fast-moving targets because of the heavier turret, more than it being at some massive disadvantage. Basically, while it'd be a good basis for future development, I can't support the destroyer right now because it's just not what we need. It's a little too big to shoot down Charybdises really successfully, and it's far too small to pose a real threat to the Tyrant.
I feel that right now a bigger gun would be overkill (and thus more expensive, slower to move, and harder to employ than it should be) for present targets, or only useful against the Tyrant, or both. The 105 is pretty good in its role, and if we maximize its performance and then move from the enhanced version to a larger cannon, we'll have an advantage in difficulty (and thus how many features and advancements we can add onto the larger gun) and have (well, you know, in that idealistic and wonderful world where things go as planned and dice rolls never hit you where it hurts) two really good guns instead of two OK guns.
Basically, our 105 is a good enough caliber for every target right now, so if we maximize its performance now, we'll be capable of destroying every target we might face for the conceivable future and we'll be set to make the jump to a larger caliber. If we go for a bigger gun, unless it's both a REALLY big gun and also capable of reaching out all the way to the Sky Tyrant despite the Tyrant's altitude advantage, it'll just be too big to shoot down the enemy airships that are a problem for us and also RIDICULOUSLY overkill for, say, ground targets like the enemy's light tank.
As for the research credit...I don't really want to spend it. If anything, I'd like to spend it on something really meaningful. The production of an airborne cruiser with a lot of new things or something, a really advanced infantry-uniform exoskeleton (since we know that those are fairly difficult and yet very useful if done right) or similar project that suffers from uniform difficulty increases but is still very, very powerful, something silly like a cloaking device, etc. A logical and probably relatively simple advancement in tech, like moving up to a destroyer-sized vessel, or improving the 105, they don't really seem worth the credit to me.