Accounting for the impact of uncaptured particles that cause compressor fouling and subsequently performance degradation when a filter system is in place is often ignored when evaluating the performance of filtration systems. Too often, the emphasis is on capture efficiency and the corresponding differential pressure loss, which are important aspects, however only constitutes a part of the overall impact on the engine performance. The main aim of this study is a first step to quantify the loss that is attributed to compressor fouling by the uncaptured particles, identify a threshold point for which further increase in pressure losses (increasing capture efficiency) no longer yields further increases in fouling levels, and subsequently investigate these respective losses and total losses in a reference high efficiency system (HES) and a hypothetical low efficiency system. Corrected operational data from a 268 MW gas turbine engine were used to evaluate the levels of degradation in the engine at different power settings. With the measured filter media pressure loss during operation and turbomatch (an in-house gas turbine performance simulation software), the impact of power reduction due to pressure loss of the filter was accounted for in the total estimated losses due to engine degradation. That of fouling was calculated based on applicable assumptions, while deducting the loss due to filtration systems from the total loss due to degradation. The study shows the inverse relationship between fouling effects and filter pressure losses as expected. More importantly, it indicates that the higher efficiency system performs better than the low efficiency system, notwithstanding the more dominant impact of higher differential pressure losses. It was also observed that the threshold where fouling effects are zero or negligible is around 800 Pa at high power setting and 600 Pa at lower power setting. In general, for all forms of the degradation using the engine data and simulation software, it is observed that at lower power settings, the impact on the engine is a lot more severe in a single-shaft constant speed operation.