Do after market sights make that much of a difference or do I just need to practice more?

I don't mean this as either rude or flippant.

In general a person with enough practice can answer this question for themselves.

I might amend my answer if I new what specific problem you were having though.

For instance, if you are shooting groups at 10 yards and trying to shrink 5" groups, then more practice.

If you are trying to hit milk jugs at 150 yards, sights might make a big difference.

Great point I sqlbullet, I notice a big difference when practicing for deer hunting with the G20 at say 25-30 yards using the Warren Tactical Sevigny sights. As for my standard S.D. training distance, say 7 yards / 21' the factory sights would work fine for many

  I'm shooting high and to the right which probably has to do more with recoil anticipation than anything. The factory sights are fine for shorter distances, but when I try to shoot beyond 20 yards the front sights cover the target and I can't see what I'm shooting at.

Aim with the center top of the front sight. Here is a handy diagram I've found on google. 

They make a difference for me if I'm shooting at a greater distance. Inside self defense ranges (7 yards) like sqlbullet mentioned it doesn't matter but stretch it to 25 yards or more and fiber optic front sights with adjustable rear sights help me shoot better.  Dawson Precision sells thin fiber optic front sights and adjustable rear sights.       

All good stuff.

Most all my "range toy" hand guns my old eyes really like the light pipe sights, my eyes settle quickly and I do make better groups, medium to rapid shots. For AR platforms, DiamondHead sights are the best FOR MY OLD EYES. Of course, carry, low light situations, etc your mileage will vary.