Jet2 are not stupid, they won't go up against Ryanair in it's own back yard on routes where they don't have a starting allocation of "protected" traffic courtesy of Jet2 holidays.
They also have little brand awareness in London so I can't see them being positioned to compete in the London-Barcelona market for a while unless, again, they can deliver a strong city breaks programme through Jet 2 holidays.
I think the competition they bring to the market will be as much with Thomas Cook and Thomson as with Ryanair. Overall therefore I think they will be a good fit for Stansted.