The Current WTT for the Bakerloo line has 6 minute turnrounds and 1.5 minute reoccupation at Elephant and Castle all day, so 8tph per platform.
https://content.tfl.gov.uk/wtt-48-bakerloo-10-sep-2023.pdf
Current Bakerloo peak service is 20tph in one direction, off peak 16tph. However the Victoria Line gets up to 36tph in the peak, and any extension would likely be looking to also go with a modernisation to increase frequencies. The Victoria line does do that 36tph with 2 platforms at Brixton (albeit with sidings beyond). The Vic line off-peak frequency according to the WTT is 135-second intervals. (26-27tph).
Assuming a 90-second re-occupation (based on current LU WTTs):
16tph, 2 plats. - 8tph/plat, 6:30 turnound
20tph, 2 plats - 10tph/plat, 4:30 turnround
20tph, 3 plats - 2x7tph + 1x6tph/plat, 2x 6:00, 1x 8:30 turnround
36tph, 2 plats - 18tph/plat, 1:50 turnround
36tph, 3 plats - 12tph/plat, 3:30 turnround
With a 2nd driver to relieve immediately, the turnround is limited by the longer of alighting/boarding or cab shutdown and startup cycle times. Equally Elephant and Castle or Lewisham could be provided with a turnback option to allow some of the 36tph to turn there instead of running all the way to Hayes/Beckenham Jn.
I would suggest that a sub-2 minute turnround would be pushing it, partly because of the conflict at the junction which would want train movements paired over it. Spreading that load over 3 platforms would give just a smidge more breathing room. Then there's the connectivity benefit of including Beckenham Jn in the tube network, and the operating benefit of not needing so many trains running if half of them don't have to go all the way to Hayes.
Brixton does run the 36tph service off two platforms with sub-2 minute turnrounds but I'm not sure if it does so utilising the sidings - the WTT Suggests not.