I have tested and tweaked again, this time I used the FLAC version of Sonic 1 as reference which was recorded from JAP Model 1 VA1 from 16bit-Audiophile website.
The older FLAC files are from PAL Model 1 VA4 which measures exactly the same JAP model.
Both can be found here:
Here is the Model 1 and Retroarch with the new HighShelfDampen.dsp settings.
Orange line is the Japanese hardware Model 1 VA1, the Blue line is Retroarch with the new filter settings.
Here is the new tweaked HighShelfDampen.dsp settings:
filters = 1
filter0 = iir
iir_gain = -4.0
iir_type = HSH
iir_frequency = 2800.0
Here is Retroarch (Blue line) unfiltered with JAP Model1 VA1:
Almost 10db difference around 10kHz area which is very much audible.
Latest release of Genesis Plus GX has built in Lowpass Filter that is closely matched to the Model1 Genesis.