Fast ion synergistic effects were studied by predictive modelling of JET best performing pulses for various levels of Neutral Beam (NB) and Radio Frequency (RF) power. Calculated DD neutron yields were analysed with the intention of separating the impact of sheer synergistic effects due to changes in fast ion (FI) distribution function (DF) from supplementary effects accompanying the application of RF power, namely changes in Te and Ti. A novel approach in analysing the efficiency of fast ions in fusion reactions based on evaluation of the cumulative reaction rates is outlined and used in the study. Conclusions on the impact of fast ion synergistic effects on fusion performance are based on comparisons of Beam-Target (BT) and thermal (Th) DD reaction rates. It was found that changes in auxiliary heating power, NB and RF, by 4MW will affect DD fusion performance and neutron rates significantly. Simulations of the best performing JET pulses show that for H minority RF heating scheme with available RF power the impact of sheer synergistic effects is somewhat lesser than the supplementary effects related to changes in Te and Ti. In conditions of much higher RF power the modification in fast ion distribution function and the impact of the fast ions on DD fusion becomes significant. The impact of the RF and NB power on the BT reactivities was found to be of similar order; however, the NB power has greater impact on reaction rates due to its larger effect on fast ion density.