Personally, I have not seen a noticeable difference in accuracy. I did some experimenting with USPSA/IPSC open division pistols using different barrel lengths. I personally like the extra sight radius with iron sights though.
Barrel length shouldn't have an effect of accuracy...... look at the 2" Model 15 vs the 6" 14....... as long as you have enough rifling to stabilize the bullet...... that "may" be impacted by bullet weight.
A bit of a hijack..... who/anyone make a red dot mount for a 9mm M&P first generation/1.0?????