No chance, I sooner have the 378s BUT with the proposed Southern 313 interior,
Failing that, FCC could refurb the 313s with the Southern interior but with FCC colours, certainly would worth it.
As to 313s being slow, I don't think they are as they accelerate quite fast as a example if you get a HST and a 313 both starting away from Stevenage at the same time, the 313 will very quickly overtake the HST but the HST's overall higher speed will let it overtake just a while longer.
Also, the reason they might seen like being slow is because of the stop start nature of their runs, you can get units go fast if you want to anyway.