Certainly a runway extension could be putting the cart before the horse.
At the moment, the only long-haul route that might be viable is US East Coast, but when that is possible in existing aircraft types, it's more important to get the route going first, then see if the extension is needed to develop it.
In the meantime, the only real affect it restricts aircraft type on potential charter routes to Cyprus and Egypt, but that is a very limited market (Are there any Cyprus charters this year?).
I don't agree that the distance to Shannon precludes US routes. Enough people travel to North America via LHR and AMS to suggest that price is the main driver of route selection.
I'd also question how long US border clearance will be provided at Shannon. It must cost a fortune compared to the number of passengers using it. In financially challenging times, it could provide an easy target.