My approach comes from a more scientific background and I am keeping it simple.

In regards to your plateau: I believe the solution which will help the most is increasing your chest day frequency. If you added in a second day to exercise chest each week, you will most likely see improvement for a few reasons: - When you do not exercise a muscle group for 6 consecutive days, you actual de-train some, ultimately reducing your coordination, strength, and the work capacity of the muscle group. Reducing consecutive non-training days from 6 to say 3 or 4 will lessen the de-training effect. - As you increase frequency of chest training days, your coordination will increase, improving your overall form regardless of your grip width. This is primarily due to increasing the neuromuscular stimulus. - You are also overloading your muscles with higher work loads than you have previously done. Naturally, your body should respond by increasing neuromuscular ability in your muscles and increasing the size of your muscles.

In regards to form (solely grip width): if you have a very wide grip, more of the weight will be transferred to your pecs. As you narrow your grip, you essentially shift more and more weight to your triceps. Since your triceps are smaller muscles than your pecs, the narrower grip will result in you being able to lift less weight. I am 6’2 with a 6’5 wing span and when I am performing bench press with heavy weight, I use a grip about 2'' wider than the outside of my shoulders. This may or may not be optimal for maximum weight, but I know I will work my pecs well and I am comfortable with this group.