There is no upper, lower, inner or outer chest muscles. There is simply the chest muscle. It contracts as a whole, not in specific areas. The reason that people seem to get better development doing inclines is because they use piss poor form on the flat bench and incorporate the front delts and triceps too much.
But for what you are asking the best exercise you could do is decline presses to the throat. Lower the bar all the way down to your chin on the decline bench. Don't touch your upper ab or lower chest like most do. Lighten the weight and bring it all the way down for 10 reps. Don't lock out at the top. 4 sets of these followed by 4 sets of cable crossovers for 12 reps on Monday. Then on Thursday 2 sets machine flyes, 2 sets incline flyes, 2 sets flat flyes. 12 reps each set.
Do that for 8 weeks (16 workouts total) and tell me you don't see results.