Meteo Data Accuracy and Adjusting Settings To Compensate

[CV219]

I've been advised by another colluegue that the meteo data (currently using PVGIS) reads too high. This is off the back of conversations with another company that own and operate and suggest that the GHI and DNI readings are overestimating from what they are seeing on their sites. I've been advised to adjust the soiling factor on the settings so that we are more in line with MCS guidlines which are more conservative. I'm now looking into solarGIS as a more accurate meteo data but for the mean time I have been requested to adjust the soiling factor. 

Question 1) Does anyone have any experience with this - I've seen some posts in the forum but it seems like everyone is using meteonorm or PVGIS - is the soiling factor the best way to make these adjustments? 

Question 2) Also when I adjust the soiling factor to 18% it doesn't seem to change the simulation results. 

Thank you :)