If you wanted to turn SNN into any form of transatlantic hub, it would be better to use a non-based aircraft.
You want the inbound to arrive between 8am and 8:30am to connect with the late morning departures and the outbound to go out between 8:30am and 9am, late enough to get everyone from the early arrivals.
If you leave from Shannon first thing, you go too early and if you don't arrive back until after 10am, it's useless for connections.