Extrusion-based metal additive manufacturing using Fused Filament Fabrication (FFF) offers a cost-effective route for producing complex metal components; however, internal defects formed during processing strongly influence the final mechanical performance. This study investigates the relationship between process-induced porosity, internal defect evolution, and tensile behavior in FFF-manufactured 17-4PH stainless steel specimens using X-ray computed tomography (CT) combined with mechanical testing. CT analysis enables nondestructive, three-dimensional characterization of porosity distribution in both green and sintered states. The results show that sintering leads to significant shrinkage and porosity reduction, while defects originating from the green parts strongly affect the structural quality of the final components. Specimens produced with optimized printing parameters exhibit more homogeneous internal structures and higher tensile strength. CT-based structural simulations accurately identify the weakest cross-sections of the specimens, with predicted fracture locations showing close agreement with experimental results. An empirical relationship between residual porosity and tensile strength is established, confirming a monotonic decrease in strength with increasing porosity. The presented approach demonstrates that CT-based evaluation provides an effective tool for linking process parameters, internal structure, and mechanical performance, supporting process optimization and quality assurance in extrusion-based metal additive manufacturing.