> " Given that my impression that higher frequency spectral data is in current calculations of radiative forcings not included is correct then are there any evaluations concerning the influence of the higher frequency spectral data on the radiative forcing of methane? Is a factor 10 for the global warming potential possible?"

I don't know.
The fact that atmospheric methane concentrations has shown changes in slope over the the years suggest that one could track correlations of the global temperature against methane. I have tried teasing signal this out with my CSALT multiple linear regression tool, but the effective CO2 seems to bury any other GHGs in the noise. That's why it's called an effective CO2 -- it acts as a leading indicator and its not a bad approximation to suggest that the other GHGs will rise and fall along with the CO2 average value.

If a big burp of methane did occur, it could have an effect, but quickly the methane would reduce to CO2 and H2O through biological activity. The jury is out whether the activation energy of methane clathrates is that much bigger than that of CO2 stores. That is the only way that methane would release faster than CO2 with rising temperatures. That's elementary physical chemistry.