Unfortunately you cant build ret's at BHX because there isn't enough room, that's why C has been built as a standard taxy way and not the ret that was initially planed. The C taxyway will help increase the movemnet rate as it will allow those going slow enough the chance to get off earlier and will greatly reduce the amount of time aircraft taxying or being towed across the airfield spend on the runway (straight across instead of F to E) and allowing departures from the Western Apron to cross and come round to E instead of backtracking. As Tesco's say, every little helps.
I think you are also wrong about using both runways simulatiously (when they do get round to building it). From memory the separation is enough to allow simultanious arrivals and departures, as is standard American practise. This wouldn't be possible with the origionaly planned longer second runway as that was too close.