Understanding the impact of micro-instabilities on the global plasma performance is essential in order to make realistic predictions for relevant tokamak scenarios. The semi-empirical transport model CGM is a useful tool to this scope because it depends explicitly on the threshold and the stiffness level, two key parameters of turbulent transport as driven by the ITG/TEM instabilities. The CGM then makes it possible to vary separately the transport variables and to see the quantitative effect of their changes on the global plasma performance. This paper focuses on the impact that four parameters (ion temperature gradient threshold, ion temperature gradient stiffness, height of the temperature pedestal and input power) have individually on the global confinement. Parameters for JET hybrid plasmas and prospective ITER plasmas are used. For JET plasmas changing the ion temperature gradient stiffness from typical low values (characterized by χs = 0.1) to high values (χs = 2) leads to variations in the H factor up to 30%. Varying the ion temperature gradient threshold within the interval of the realistic values 3–7 causes changes in H98 between 20% and 30%. The effect of the temperature pedestal height is very considerable (over 50% of H98 variation changing the temperature pedestal height from 1 to 3 keV), in agreement with the previous investigations. H98 is found to slightly decrease with increasing power (from 20 to 60 MW of injected NBI power) for high stiffness and to remain constant in the case of low stiffness. For ITER plasmas the variation of the H factor has qualitatively similar trends, but the variations with respect to changes in stiffness and threshold are smaller. However, very important changes are found for the values predicted by the fusion power in these plasmas.