Strange that you would ask that. Early this season I sent out some emails and looked closely around for stories relating to this. The reason for my curiosity was one of vision and space. Those of us who have donned helmets can attest that they tend to restrict perepheral vision (especially full face helmets) and also find that they tend to bump into everything around you. It is amazing how much bigger they seem to make your head!
What I gathered is that prior to the season as the pit crews practiced wearing the helmets times suffered, but as the crewmen became accustomed to them, no real effect was realized.
Some crewmembers still complain of restricted vision, though just as many report the lack of perepheral vision actually increases their focus by removing external distractions.
All the teams track their pit times very closely, but to my knowledge the info is not made public. As a general observation the answere would remain no. We see pit times in the 14 second range frequently with some dipping into the 13's. I would contend that in some cases a pit stop delayed by some other factor such as malfuntioning wrench, a hung air line, a failed jack, or even a runaway tire are delayed further because of the "isolation" a helmet creates. This is strictly an opinion thing on my part.
In a few weeks when the pit crew championships are held at Rockingham we might hear more about this subject.