1) Shooting into combat and if people get in the way of your target netting you an effective -8 to hit is still very much a problem, so the -6 on top of that will almost certainly cause a flurry of misses.
2) I think you're overlooking that you need a free hand to reload your guns, which is the main obstacles to Guns Akimbo type builds. That would somewhat preclude getting 6 shots a round at level 3, let alone being able to afford a pair of double-barreled pistols by then. Also the fact that having double barrels doesn't mean you have the attacks to use them all. If you try to shoot both as one attack, you have an extra -4 penalty to your attacks. Not to mention that Rapid Shot only gives you one extra shot, not one extra per weapon.
I'm not saying Rapid Shot isn't a bad idea, it's just that you'll have adequate numbers just dual-wielding and troubles keeping up with the reloading without having to worry about Rapid Shot on top of it. It works out nicely for single firearm wielders, though. I also think that pistols are weighed equivalently to light crossbows that you could get away with calling them light weapons for TWFing purposes. It doesn't say anywhere what they're treated as but it's what makes the most sense to me.