Thanks for this. So... a third of the island platform is sliced off to make way for the bay and the 'down' Crosscity line uses P2. This allows the ten-minute interval service to be re-instated and extra paths for fast trains on the Camp Hill line. P4 can then be taken out of use.
The down-side is the Camp Hill stoppers would just run from Moor St to Kings Norton. This doesn't bode well for the new service as it's sort of stymied forever (although theoretically you could have Stourbridge-KN locals).
Like others have argued, a new bi-directional P3 through line (instead of the bay) would offer more flexibility. 'Down' locals from both the Crosscity and Camp Hill lines could be offered a refuge, allowing fast trains to overtake from either line. The 'locals' from Moor St could potentially have a more extensive reach to Longbridge or beyond.
I want the new stations to be a success, and hope it all works out.