AIWB, not really. See BBI's post above about the 3" Wiley Clapp versus the 4" Match Champion. My experience is skinny barrel 4" Model 10 and 12 versus 2" model 12's (fixed sight K frames respectively in steel and aluminum frames). Less difference than you would think. In some positions the 4" is actually more comfortable than the 2". A heavier barrel might change that but skinny barrel is limit of my experience for AIWB. Heck, for old school, there is even a Skeeter Skelton article about barrel lengths and comfort that says as much. I think I have it bookmarked somewhere...
ETA: yep, here's the article:
http://darkcanyon.net/The%20Long%20&...%20Barrels.htm