from personal experience, i can say that it absolutely changes point of impact. the block itself i think has very little to do with it. it is the amount of gas that will have a great effect on point of impact.
to prove it to yourself, take a rifle with an adjustable gas block and set your zero. then close the gas block 1/2 turn at a time and shoot groups at each setting, then go back the other way and open it up all the way. on 3 different rifles i have had point of impact changes up to 2" at 100 yards, on all the rifles, the change was significant. one was a 24" barreled ar-10, the other two were ar-15's in 223.