Brain-computer interfaces (BCIs) have made remarkable progress in recent years, driven by advances in neuroscience and clinical applications. For practical use, underlying processing systems must meet strict latency and power budgets. However, existing BCI systems typically rely on a single processing node to handle the entire workload, making it difficult to satisfy these budgets across diverse applications. In this work, we present TierX, the first simulation framework for design space exploration of multi-tier BCI systems. TierX models heterogeneous processing nodes across tiers, including implanted processors, body-attached devices, and external servers, together with diverse communication and powering methods. It navigates the extensive design space to identify optimal (1) workload partitioning options and (2) system configurations that leverage the strengths of each tier. We validate TierX on representative system configurations and demonstrate its effectiveness across diverse use cases.