I dont have a clue about how much memory Cell will need exclusive but thinking of TurboCache and the FlexIO there may be ways to get "extra" bandwidth i speculate.
For example if you lock 64/128MB of XDR memory to be used as texture and "swap" textures beetwen the GDDR3 it seems that you could "earn" a 10GB(+/-) of bandwidth and still have quite plenty of both read/write from FlexIO bus left.
We seen many tricks this gen of games so it shouldt be that hard to do.