How much left and high? I think I would try adjusting the rear over to the right a bit and see what I got then. Elevation could just as easily be the ammo you are using if you are at 15yd. or beyond. At closer ranges that may not be the issue. If you are shooting at 7yd. or less, then the problem is almost certainly you.

Follow through is very difficult to teach with the handgun, yet extremely important. The easiest way I have found to remind new shooters about it is to have them make certain the sights stay on the target for at least 3 seconds after the shot is fired. (Ye, I do know about recoil, and no, you don't do this in advanced shooting...but you DO stop the gun on each target, and you do not move the gun before the bullet is out of the barrel)

Do not waste a bunch of time wondering "why"...there may be several reasons for the difference between the two guns...and remember, we are only speaking about two, individual pistols, so it would be a really long leap to try and generalize the issue. Still, one slight possibility is that the longer barrel of the G17 allows for longer "bullet dwell" assuming the same ammo, so IF you are a bit sloppy with your follow through, it may be that you are moving the barrel of the 17 before the bullet exits, while the bullet from the G19 is gone before you do the same with it. This is just speculation, of course, and the slight difference in velocity between the two barrels may, or may not act to equalize the amount of bullet dwell, etc. if there IS any significant difference.

Yes, the rear sight can be drifted in the dovetail cutout. Take the slide off and lay it on its side on top of a roll of duct tape and use a brass punch and a hammer or have someone with a sight pusher move it for you. Elevation can be adjusted by using either front or rear sights of different heights.