Bullseye, no, you have it backwards. If you are shooting high and your rear sight is all the way down, you need to go to either a faster or lighter bullet (or both.) Look at it this way. When you line your sights on the target, the muzzle of your gun is actually below your aiming point. When you pull the trigger, the gun immediately starts to rise. When the bullet leaves the muzzle, as long as you are sighted in correctly, the muzzle will be at the right height for the bullet to strike your aim point. If you shoot a faster/lighter bullet, the bullet will get out of the muzzle faster, not giving the muzzle the same time to rise to the same point as with the heavier/slower bullet, resulting in a lower hit on the target.
I had a similar thing happen to me this year. I bought an 870 rifled slug barrel and some 2 3/4" sabot slugs. (No need to beat myself up!) With the rear sight cranked all the way down, I was hitting over 9" high at 50 yards - unacceptable. I then tried some 3" sabot slugs. They had the same slug weight, but were moving faster because of a larger powder charge. I cranked the rear sight up a few clicks and after a few shots, had it shooting right where I wanted it.
Make sense to you?