Of course Stobart didn't have the choice of extending the runway by more than 300m as a Code C runway cannot have a TODA or ASDA of more than 1,799m and its location means that it will always be Code C. So it was a case of making the best of what they had to work with while anticipating it would be just sufficient to attract the necessary business.
I agree with fatmed that the new generation of mid-size airliners have better runway performance and that does open up more potential business for SEN.