Not if you ask Guinness. Their FAQ states:
Is it true that you get a much better pint in Ireland? For GUINNESS® brewed outside Ireland the water will be different of course because we always use pure, fresh water from local natural sources. In blind tests though, (with a bunch of highly cynical journalists!), none of our sample could tell the difference. That said, all GUINNESS® sold in North America is brewed in Ireland at the historic St .James's Gate Brewery in Dublin. So, the only real difference you should be able to spot is the pub that you're drinking in!
I can believe the serving conditions are certainly different in most US pubs. A lot chill it like it's a Bud and kill all the flavor. Of course Guinness themselves have been promoting some of this with their Guinness Draught Extra Cold. There is also an Extra Stout that was common in the US which is higher in alcohol, but that appears to be slowly disappearing in favor of Draught in Can/Bottle (an oxymoron if I ever heard one).
Andy