You would think that a heavier bow would shoot a flatter trajectory than a lighter bow, but a heavier bow requires a heavier arrow, which partially offsets the effect of the heavier draw weight. I've heard that you can generally expect higher arrow speed for a typical bow and arrow setup until the bow reaches about 60#, with not much if any improvement after that. The main reason people want to shoot a heavier bow is to gain increased penetration when hunting.
I also understand that the average bow weight used by an Olympic archer is about 39#, and they are shooting at distances around 100 yds. They shoot very light arrows, of course.
Any arrow starts dropping the moment it leaves the bow, when shot level to the ground. When you say "straight shot aim," you probably mean whatever elevation it takes to hit the target at 20 yards or so, which may seem pretty straight compared with the elevation it takes to hit the target at 50 yards. If you hold any bow, of any weight, at the same elevation it took to hit the target at 20 yards, it will hit low at 25 yards.