I'm pretty sure that opp fire is a bit TOO overpowered since it triggers a shot if you spend even 1 AP. I'm pretty sure I can turn my head before a unit gets a chance to aim and fire a shot. My idea is to make guns have an initiative value which is how many APs an enemy unit(s) can spend before that gun is fire from opp fire. It'll nerf opp fire and be another element you can use to rebalance guns in the game.
I like what you say about how different weapons handle. I probably meant if you play the game a lot the current set up works very well. So if play more you will be more concerned with tactics and battle situations than focusing on awkward aspects of the game. The biggest factor is the human element. There are so many ways of dealing with oppertunity fire. I don't massively disagree with your points, just the degree of issue.
That could work too, but there's indoors where that won't really affect gameplay. Also, petermock, it could help to avoid stalemates where both players are waiting with high accuracy overwatch weapons for the other to move so that they can shoot.
Another idea I had is reduce the accuracy of overwatch in comparison to aimed shots so that players end up relying on it more. You could also have it so that some weapons could only fire off 1 or 2 bullets versus others which can do many (e.g a sniper versus an assault rifle). This could be combined with burst firing mechanics (e.g an assault rifle or chaingun [DO IT STEVE!] can fire multiple low damage bullets at once).
Personally I hate opp fire, it leads to defensive, hide in a corner style victories. It may be less accurate but it is still pretty lethal, even over ridiculously long distances :( I'm all for anything that reduces its effectiveness :)