The Sigma is plenty accurate to 20 yards and beyond, assuming you've started at 7 yards and worked your way back, slowly with patience. Shooting is the same as any other activity. You need to take your time to learn how to do it properly, correcting errors as they come BEFORE you move on to more difficult facets, like longer distance shooting.
Consider practicing WITHOUT using the sights by using instinct from 5 yards. Pick up the gun and WITHOUT using the sights, fire 5 shots at the target. If you aren't hitting at least 8s 9s and 10s, you may want to work on that before you even consider using the sights, to hone your natural abilities.
I'm not sure what your ultimate goal is with your firearm, but if it is home defense, consider that much of the time when a burglar wakes you and you go to investigate, it takes far too long to use your sights, especially if HE is armed as well.
I suggest using primarily the front sight when shooting without lining up the sights. Your instincts and natural abilities should allow the gun to shoot where you're pointing it without looking if you've practiced it enough.
I'm not in any way an expert... Just making suggestions based on observations I've made over the last 8 months of owning my Sigma 9mm. Yes, the trigger is heavy and long. Get used to it, get GOOD with it, and every other semiauto you pick up you'll be shooting straight 10s.
Took my girl to the range a couple months ago and she was shooting a .22 Ruger. After shooting about 80 rounds through my Sigma, I picked up Ruger and without using the sights blasted 9 rounds straight through the 10 at 7 yards.
You'll be surprised how much better honing your natural abilities will aid your sight usage, once you understand how YOU'RE reacting to the gun.
/rant