That said, I usually take my shots around 200 range (icon indication) and I would like to be able to shoot my guns in unison. In this case is it still useful to have 375 convergence, or would I be better served setting both the MGs and the 30 down to around 200 conv?
What's the point of setting your convergence at all? Or knowing where it's set? If you set your convergence at 375, how do you know when you're 375yds from your target? The point of adjusting and/or knowing your convergence point is to give you a predictable firing distance to hit and do the most damage to your enemy.
It's not possible to know when you're 375 yards from your target in AH, unless you memorize the size of each of your opponents, and how they compare in size in your sights. The icons in the game won't tell you when you're 375 out.
The useful (IMO) enemy icons for gunnery in the game are D200, D400, and D600. Each icon range encompasses a span of 200yds, with the depicted distance being the CENTER of that span. So, D200 is the space between 100 and 299yds. D400 is anywhere between 300 and 499yds. D600 is anywhere between 500 and 699yds.
If you set your convergence at 375, how do you know you're 375yds from your target? You don't. You have to guess (and you may even get good at it), because 375 is "somewhere" around 1/2 way from when the icon distance switched from D400 and will switch to D200. It's not very predictable, since speed/closure rates will vary so much. 375 may be the perfect compromise in trajectory (I'm not saying that's true), but what good is it if you never know when your target is 375yds away?
So, how do you use the icons to give you a better reference of actual firing distance? There are a few "sweet-spot" distances that you can use, and actually know when you are very close to firing at your pre-set convergence distance. This maximizes the chances that you'll actually land a tight group on your target, or that you'll hit the target with one well-placed big-azz round. Using these sweet-spot convergence ranges, and firing at the correct time, takes the trajectory issue out of play for the most part, and let's you think more about properly timing the trigger-pull.
So, what are the sweet spots? They're the distance that one icon changes to the next icon distance. The icon will change from D400 to D200 (or vice versa) at 299 or 300yds. It will change from D600 to D400 at 499 or 500 yds. If you set your convergence to 300 or 500, and fire when the icon counter changes, it's ridiculous how easy it is to destroy a plane with a tiny burst. My normal is to set mine for 275, and while the icon says 400, I'm getting my aim fairly close. As soon as it switches to D200, I finalize my aim and fire, and I'll be firing at my opponent right around 250-275 yards. I don't use 500 for convergence, because from my testing I've found that the normal dispersal of the rounds at 600yds isn't really any worse with a 275 convergence setting than it is with a 600yd convergence setting, and for me, fighters out at D600 are too far away (and too small) for me to shoot at. 275 optimizes my effectiveness on fighters, but doesn't hurt my performance against bombers, which I'll fire at sooner (about D400 usually). In crossing shots (high deflection and speed), your chances of success are much higher against a closer, larger target with less required lead, which is another argument for a 275 setting vs a 475 or 500yd setting.
These guys have the "let the ACM do the work" idea just right. If you do that, and have your convergence set right, and learn to fire at convergence distances, then the actual firing part of the fight is quick, small, and over quickly. Winning the fight is more about winning the flight aspect, and it's just finalized with a brief shot.