What an enlarged Type 45 should gain in missile capacity

Would a Type 45 enlarged to 11,000-12,000 tons for 64 Sylver A50 cells and later 48 Sea Ceptor cells suffer from the extra displacement?

No; the larger hull should use its displacement for a much deeper missile battery rather than stop at 64 cells.

Extra displacement is useful when the design converts it into additional capability. The answer therefore expects a roughly 12,000-ton Daring derivative to carry substantially more than 64 VLS cells, suggesting an initial target of around 144 cells. Possible arrangements include two 64-cell Sylver A50 groups with additional Mark 41 launchers. The 48 small launch cells proposed for Sea Ceptor would occupy comparatively little space. The launcher combinations change during the discussion and remain provisional layout suggestions.