It certainly seems a win win situation for Ryanair to operate into EXT (and CWL). Taking Malta as an example the new flights from EXT and CWL have been well loaded as have those from BRS with a lot of sold-outs at the latter. The same thing applies with EXT and Malaga. Ryanair has also been adding flights from BRS on some sun routes this summer so they seem to have covered all the South West Britain bases.
Away from the sun, Ryanair thus far seems content to begin new routes at BRS at the moment, rather than try them at EXT (or CWL) instead, even though recent arrivals compete with easyJet such as Seville, Milan MXP (although they got it there before easyJet announced theirs), Sofia (ski season only).
BRS gets most of its non-core catchment passengers from South Wales and Devon. Received wisdom seems to have been that a lot of thinner routes (not just with Ryanair) in South West Britain go to BRS because it has a larger core catchment than either EXT or CWL with BRS's physical location also enabling it to 'top up' from both CWL and EXT catchments. Ryanair's moves on this in the coming years will be interesting to see if it believes that EXT can sustain any of its non-sun routes.
To comment on the point about a lot of BRS passengers coming from the southern BHX catchment, it's not born out by the stats which means that South Wales and Devon have the most BRS passengers to try to re-capture, or capture for the first time with some of them.