GJ -- I also have the Sevigny sights on my G17. Had the same problem with 120 grain LRN and 4.0 grains Winchester 231 (a VERY slow load).
You need a taller front sight or a lower rear sight to lower the POI with the same ammunition.
Heavier bullets may or may not lower the POI, depending on velocity. Slower velocities generally have higher POI for any given bullet weight (at pistol ranges), at least that's my experience.
I had good luck getting POI=POA at 25 yards by speeding up my load a little. I'm now using 4.4 grains W231 (book maximum for this powder bullet combination).
You might also experiment a little to figure out how much to take off the rear sight. A little masking tape will help you find the right height. It won't take very much to change the POI.
Here's and example of how to determine the amount of metal to remove to shift your POI (Shamelessly copied from Brownells)
Correcting Sight Height
If your rifle or pistol is shooting high or low, there is a formula you can use to determine what the correct height for your sight should be.
Terms
Amount of Error: Measure in inches the distance from the bullseye to your shot.
Sight Radius: Distance in inches from the front sight to the rear sight. *
Distance to Target: The distance in inches from your shooting position to the target.
Note: Be sure that your rear sight is at its mid-point before you shoot.
Formula
Amount of Error X Sight Radius = Sight Correction Needed
Distance to Target
If you are shooting low, you would replace your front sight with a lower sight, and conversely, if you were shooting high, then you would replace your front sight with a higher sight. Add or subtract the number of thousandths needed to the actual height of your sight and you will have the correct height of the replacement sight.
Example
At 100 yards, your shot is 6" low and the distance between your front and rear sight is 19.5".
6 X 19.5 = .0325
3600
Since you are shooting low, you would need to lower your front sight by .032".
So for your pistol, it seems that
(3.5 incles low x 6.75" sight radius)/(20 yards x 36 inches/yard) = 23.625/720 = 0.0328 inches.
Lower your rear sight by 0.03 inches and you should be pretty close. A smooth file, blue magic marker, a steady hand and a brave heart will get you there. Touch up with cold blue or a little rust blue.
Good luck