Although you single out SOU as if it is 'holding back' the rest of the network don't forget that a number of flybe's other key bases with shorter runways:
BHD - 1829m
JER - 1716m
GCI - 1463m
IOM - 1837m
SOU - 1723m
Although the mentioned bases are a minority in the bigger scheme of things you have to remember SOU is their largest base and I belive BHD is their second largest. JER is one of the larger bases and both GCI and IOM have a fair amount of Flybe's operations. These bases could all benefit greatly from the E175's being added, remember all the hoo-hah about the new aircraft being introduced on LGW-GCI? The best we can hope for is either a future order or the conversion of some of the existing order so Flybe's will get the most out of their shiny new aircraft while still benefitting to some extent their fuel costs thanks to the hairdryers!