As far as new guns go, rarely have I not had to do some polishing or touching up to get them to function perfectly. I'd expect a little work on an 870 as I would any other...just strip it down, give it a good cleaning, smooth out any rough patchs and then take it shooting. The 3.5 "should" work just as well as the 3.0 model.
As for the utility of the longer chamber, I can't imagine living without it. As a guy who hunts in situations where close shots are rare I feel I absolutely Must have the 3.5 chamber for waterfowl. But that is because I use steel shot and it is a truely pathetic substance to use as a projectile.
If I were still firing lead, or if I had the inclination to buy shells for $4 each and use the new "heavy as lead" (or heavier) metals, then I doubt there would be many situations where a 3" chamber would not be plenty.