This paper presents a comprehensive performance analysis of the BullSequana eXascale Interconnect (BXI) using the \textit{Portals4} programming model. The main contributions include: (1) the design and implementation of PtlBench, a Portals4 microbenchmark suite that evaluates low-level features such as bandwidth, latency, cache effects, and triggered operations; (2) a detailed comparison of Portals4-compatible MPI implementations (OpenMPI, ParaStationMPI) and our custom Portals4 device for the PGAS library GPI-2, covering point-to-point, one-sided, and collective operations; and (3) application-level analysis using the Himeno and SSCA1 benchmarks to assess the impact on different communication patterns. These results provide valuable information of BXI's capabilities and limitations for real-world HPC workloads and communication models.